MODULE 1 SURVEY OF THE PLANT KINGDOM
Module Structure
In this module we will discuss about the cellular organization, structure and functions
Unit 1 Kingdom Plantae
Unit 2 The Seedless Plants
Unit 3 Bryophytes
Unit 4 Seedless Vascular Plants
Unit 5 Seed Plant I: Gymnosperms
MODULE 2 OVERVIEW OF THE PLANT KINGDOM
Unit 1 Kingdom Plantae
1.1 Introduction
1.2 Intended Learning Outcomes (ILOs)
1.3 Main Contents
1.3.1 The Plant Kingdom
1.3.1.1 Classification of the Plant Kingdom
1.3.1.2 The Diversity of Land Plants
1.3.2 Seedless Vascular plants
1.3.2.1 Algae and Evolutionary Paths to Photosynthesis
1.3.2.2 Plant Adaptations to Life on Land
1.3.3 Alternation of Generations
1.3.3.1 Sporangia in Seedless Plants
1.3.3.2 Gametangia in Seedless Plants
1.4.3.3 Apical Meristems
1.4 Summary
1.5 Self-Assessment Exercises
1.1 Introduction
A diverse array of non-seed-bearing plants graces the terrestrial landscape. Mosses might find their place on a tree trunk, and horsetails may stretch their segmented stems and delicate leaves across the forest floor. Although seedless plants constitute a small portion of our present day plant life, about 300 million years ago, they dominated the scenery, thriving within the vast, marshy forests of the Carboniferous epoch. Their decomposition contributed to the formation of substantial coal deposits that we now extract. Contemporary evolutionary theory asserts that all plants, including certain types of green algae along with terrestrial plants, share a common ancestor—a monophyletic origin. The transition from aquatic to terrestrial environments presented significant challenges for plants. They needed to develop mechanisms to prevent desiccation, disperse reproductive cells in the air, provide structural support, and efficiently harness sunlight. While seed plants have evolved adaptations enabling them to inhabit even the most parched habitats on Earth, complete emancipation from water was not universally achieved among all plants. The majority of seedless plants still rely on a moist environment for their reproductive processes.
1.2 Intended Learning Outcomes (ILOs)
By the end of this unit, you should be able to:
1. Describe the characteristics features of the plant kingdom
2. Describe the criteria and the classification of the plant kingdom
3. Explain the diversity of the plant kingdom
4. Discuss the challenges to plant life on land
5. Describe the adaptations that allowed plants to colonize the land
6. Describe the timeline of plant evolution and the impact of land plants on other living things
1.3 Main Contents
1.3.1 The Plant Kingdom
All the plant life present on our planet falls under the classification of Kingdom Plantae. This kingdom encompasses autotrophic, multicellular, photosynthetic eukaryotic organisms. These entities are primarily non-mobile, capable of forming embryos, and serve as the primary producers in ecosystems due to their capacity to convert solar energy into chemical energy through photosynthesis. They possess a firm cell wall composed largely of cellulose. The Kingdom Plantae encompasses a diverse array of organisms, with over 300,000 documented species. Among these, more than 260,000 are categorized as seed plants. Mosses, ferns, conifers, and flowering plants all belong to the plant kingdom. The emergence of land plants occurred within the larger Archaeplastida group, which also includes red algae (Rhodophyta) and two categories of green algae: Chlorophyta and Charophyta. While some biologists consider certain green algae as part of the plant kingdom, there is debate over this classification. This divergence in opinion arises from the fact that only specific green algae, the Chlorophytes and Charophytes, share key traits with land plants, such as utilizing chlorophyll a and b alongside carotene in the same proportions as plants. These attributes are absent in other types of algae.
1.3.1.1 Classification of the Plant Kingdom
Kingdom Plantae is also referred to as the Kingdom of Plants. The five kingdoms—Monera, Protista, Fungi, Animalia, and Plantae—that Whittaker proposed as a general classification of living things in his system from 1969. The kingdom of Plantae includes all living things. All living things are multicellular, autotrophic eukaryotic creatures. Cell walls are one of the primary properties of plants. The cell wall gives the cell rigidity and structural stability. A stiff cell wall is present in each plant cell. Chloroplasts and the pigment chlorophyll are present in plants, which are necessary for photosynthesis. Subgroups are used to further categorize the plant kingdom. Classification is based on the following criteria:
1. Plant body: Presence or absence of a well-differentiated plant body. E.g. Root, Stem and Leaves.
2. Vascular system: Presence or absence of a vascular system for the transportation of water and other substances. E.g. Phloem and Xylem.
3. Seed formation: Presence or absence of flowers and seeds and if the seeds are naked or enclosed in a fruit.
Each subgroup of plants has a unique collection of identifying traits that are unique to that subgroup. The angiosperms are plants with a complicated assembly, a circulatory system, and a reproductive system that are well-established in their own families, whereas thallophytes are the greenest of the plants. The plant world has been classified into five groups by taking into account all of these traits. They go by the following names: The first of these is the Thallophyta, followed by the groups of organisms Bryophyta, Pteridophyta, and Gymnosperms and Angiosperms, which are all types of flowering plants.
IN-TEXT QUESTION (ITQ): Which members of the plant kingdom is the greenest? The thallophytes are the greenest of the plants.
1.3.1.2 The Diversity of Land Plants
Land plants are categorized into two primary divisions based on the presence or absence of vascular tissue, as outlined in Figure 1.4. Plants that lack specialized cells for transporting water and nutrients, collectively known as vascular tissue, are classified as nonvascular plants. Examples of these are bryophytes, including liverworts, mosses, and hornworts, which are seedless and devoid of vascular tissue. These plants are believed to have emerged early in the evolutionary timeline of land plants. Vascular plants, on the other hand, possess a system of cells designed to transport water and solutes throughout their structures. The initial vascular plants likely appeared during the late Ordovician period (461–444 million years ago) and were possibly similar to lycophytes, which encompass club mosses (distinct from actual mosses) and pterophytes such as ferns, horsetails, and whisk ferns. Lycophytes and pterophytes are collectively referred to as seedless vascular plants, signifying their lack of seed production. Seeds are embryonic structures protected by a tough covering and containing stored nutrients. Among all plant groups, seed plants constitute the largest, dominating the landscape. This category includes gymnosperms, notably conifers that produce "naked seeds," and the highly successful flowering plants, or angiosperms, which safeguard their seeds within chambers located within flowers. Over time, the walls of these chambers evolve into fruits.
Figure 1.1 Diversity of Kingdom Plantae: Source: https://www.studocu.com/
Self-Assessment Exercises 1
1. What is the diversity of plants?
2. What is the most diverse group of the land plants?
1.3.2 Seedless Vascular plants
There are numerous and diverse groups of organisms in the kingdom Plantae. The number of cataloged plant species exceeds 300,000. More than 260,000 of these are seedlings. The plant kingdom includes ferns, conifers, flowering plants, and mosses. The Archaeplastida, which also includes the red algae Rhodophyta and the two classes of green algae Chlorophyta and Charaphyta, is where land plants first appeared. Most biologists agree that at least some green algae are plants, however some do not consider any algae to be part of the plant family. The only green algae, the Chlorophytes and Charophytes, share traits with land plants, such as utilising chlorophyll a and b plus carotene in the same proportion as plants, which is the basis for this argument. These characteristics are absent from other types of algae.
1.3.2.1 Algae and Evolutionary Paths to Photosynthesis
There are differing opinions among scientists regarding the classification of algae within the kingdom Plantae. Some argue that all algae should be considered plants, while others contend that the Plantae kingdom should encompass only green algae. Another perspective is to include only Charophytes as plants. These contrasting viewpoints stem from the diverse evolutionary pathways that different forms of algae have taken to achieve photosynthesis. Despite all algae being photosynthetic, possessing some form of chloroplast, the development of this capability varied. Approximately 1.65 billion years ago, the ancestors of the Archaeplastida engaged in an endosymbiotic relationship with a green, photosynthetic bacterium. The subsequent descendants from this lineage, such as red and green algae, ultimately gave rise to present-day mosses, ferns, gymnosperms, and angiosperms. This evolutionary trajectory followed a mostly linear and monophyletic path. On the other hand, brown and golden algae within the stramenopiles group, along with other algae beyond the Archaeplastida, acquired photosynthetic abilities through secondary or even tertiary endosymbiotic events. This involved the incorporation of cells that already contained endosymbiotic cyanobacteria. While these latecomers to photosynthesis are similar to the Archaeplastida in terms of autotrophy, they did not colonize or spread across the Earth to the same extent.
In summary, the classification of algae as plants is a subject of debate due to the various ways in which different types of algae have evolved photosynthetic processes. This diversity is linked to the manner in which their ancestral lineages acquired photosynthesis-related traits.
1.3.2.2 Plant Adaptations to Life on Land
The transition to terrestrial life posed numerous challenges for organisms, necessitating adaptations to cope with the new environment. Water, vital for life, becomes scarce on land, leading to the risk of desiccation. Even in proximity to water, aerial plant parts are vulnerable to dehydration. Buoyancy is absent, prompting the need for structural support. Additionally, the lack of water as a filter exposes organisms to harmful radiation, including ultraviolet rays. Reproduction faced hurdles, requiring novel strategies for male gametes to reach females without water assistance. This shift demanded protection against desiccation for both gametes and zygotes. Land plants devised strategies to tackle these challenges, although not all adaptations emerged simultaneously. Despite these difficulties, terrestrial life offers advantages. Sunlight is abundant, offering energy for photosynthesis. Carbon dioxide diffuses faster in air, providing a ready source. The absence of land animals initially spared plants from predation, but defensive mechanisms like thorns and toxins evolved as animals adapted. Early land plants and animals, staying close to water, developed strategies like tolerance to drying. Others colonized humid environments or used desiccation resistance. Successful land adaptation involved developing advantageous structures. Four key adaptations facilitated plant success on land. Alternation of generations, involving sporophyte and gametophyte stages, was present in all land plants. Apical meristems in roots and shoots enabled continual growth. A waxy cuticle resisted desiccation, while cell walls with lignin provided support. These adaptations contributed to land plant success, prompting debates on the placement of closely related green algae within the plant kingdom. Mosses, lacking some adaptations, represent an intermediate stage in land colonization. What are the 4 land plants?
Self-Assessment Exercises 2
What is the meaning of land plants?
2. What are the two main groups of land plants?
1.3.3 Alternation of Generations
All sexually reproducing organisms have both haploid and diploid cells in their life cycles. In organisms with haplontic life cycles, the haploid stage is dominant, while in organisms with a diplontic life cycle, the diploid stage is the dominant life stage. Dominant in this context means both the stage in which the organism spends most of its time, and the stage in which most mitotic cell reproduction occurs—the multicellular stage. In haplontic life cycles, the only diploid cell is the zygote, which undergoes immediate meiosis to restore the haploid state. In diplontic life cycles, the only haploid cells are the gametes, which combine to restore the diploid state at their earliest convenience. Humans, for example, are diplontic. Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages. This type of life cycle, which is found in all plants, is described as haplodiplontic
Figure 1.2 Alternation of generations between the 1n gametophyte and 2n sporophyte. Mitosis occurs in both gametophyte and sporophyte generations. Diploid sporophytes produce haploid spores by meiosis, while haploid gametophytes produce gametes by mitosis. Source: https://openstax.org/books/concepts-biology/pages/14-1-the-plantkingdom
The developmental sequence is followed by a multicellular diploid form called a sporophyte after the multicellular haploid form known as a gametophyte. The gametophyte undergoes mitosis to produce the gametes (reproductive cells). This stage of the plant's life cycle can be the most visible, as it is in the case of mosses, or it can take place in a tiny structure, like a pollen grain, as is the case with seed plants. The sporophyte generation has become more prominent as terrestrial plants have evolved. When it comes to non-vascular plants, which include mosses and liverworts, the sporophyte stage is seldom evident. The sporophyte phase in seed plants can develop into a massive tree, as in the case of sequoias and pines. Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered from desiccation and other environmental hazards. In both seedless and seed plants, the female gametophyte provides protection and nutrients to the embryo as it develop into the new sporophyte. This distinguishing feature of land plants gave the group its alternate name of embryophytes.
1.3.3.1 Sporangia in Seedless Plants
The seedless plant's sporophyte, which is diploid and created by the syngamy (fusing) of two gametes, lacks seeds. The sporangia (plural: sporangium) are carried by the sporophyte. The word "sporangia" literally translates to "a vessel for spores," since it refers to the reproductive sac in which spores are produced. It is important to note that in many plants, polyploidy causes chromosome number to be complicated; for instance, durum wheat is tetraploid, bread wheat is hexaploid, and some ferns are 1000-ploid. In the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, during which the 2n chromosome number is reduced to 1n. Later, the sporangia release the spores, which disperse throughout the environment. The haploid spore undergoes mitosis to produce a multicellular gametophyte when it germinates in a favorable environment. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte (vegetative form). The cycle then begins anew.
Figure 1.3 Sporangia. Spore-producing sacs called sporangia grow at the ends of long, thin stalks in this photo of the moss Esporangios bryum. Source: https://openstax.org/books/concepts-biology/pages/14-1-theplant-kingdom
Plants that generate only one type of spore are termed homosporous, resulting in a gametophyte that produces both male and female gametes, typically on the same organism. This is observed in non-vascular plants where the gametophyte stage dominates the life cycle. Conversely, plants that yield two types of spores are referred to as heterosporous. The smaller male spores, known as microspores, develop into the male gametophyte, while the larger megaspores transform into the female gametophyte. Heterospory is observed in a few seedless vascular plants and all seed plants, with the sporophyte phase being dominant. The spores of seedless plants are encased in robust cell walls containing the resilient polymer sporopollenin, which is also present in pollen grain walls. This intricate substance is recognized by its elongated chains of organic molecules resembling fatty acids and carotenoids, giving pollen its yellow hue. Notably resistant to chemical and biological breakdown, sporopollenin's durability accounts for well-preserved pollen fossils in seed plants, where pollen functions as the male gametophyte. Previously attributed to land plants, sporopollenin is also found in the charophyte Coleochaetes.
1.3.3.2 Gametangia in Seedless
Plants Multicellular haploid gametophytes have structures called gametangia (plural: gametangium). Precursor cells undergo mitosis to produce gametes in the gametangia. Sperm are released from the male gametangium (antheridium). Seedless plants create sperm with flagella that can swim to the female gametangium, the archegonium, in a moist environment. As the sporophyte, the embryo grows inside the archegonium. In contrast to seed plants, which lack or have rudimentary gametangia, seed plants are dominated by them.
1.3.3.3 Apical Meristems
In a region known as the apical meristem, a small mitotically active zone of cells present at the tip of the shoot or root, plants' shoots and roots grow longer by fast cell division (Figure 1.4). Undifferentiated cells that continue to divide throughout the plant's lifetime make up the apical meristem. All of the organelle's specialized tissues are derived from meristematic cells. A plant can access more area and resources by growing its shoots and roots longer, including light in the case of the shoot and water and minerals in the case of the roots. Tree trunk diameter-increasing cells are produced by a distinct meristem known as the lateral merist
Figure 1.4 Apical meristem at a root tip. Source: https://openstax.org/books/concepts-biology/pages/14-1-the-plantkingdom
Addition of new cells in a root occurs at the apical meristem. Subsequent enlargement of these cells causes the organ to grow and elongate. The root cap protects the fragile apical meristem as the root tip is pushed through the soil by cell elongation. What is haplodiplontic in the life cycle of a plant?
Self-Assessment Exercises
What is Alternation of generations?
What is apical meristem?
Summary
The terrestrial realm showcases a remarkable diversity of non-reproductive plants. Mosses can be found on tree trunks, and horsetails may spread across the forest floor with their segmented stems and thin leaves. However, around 300 million years ago, the lands cape was dominated by seedless plants, flourishing in the expansive swampy forests of the Carboniferous era. Today, seedless plants constitute a smaller portion of the plant population in our surroundings. These plants significantly contributed to the formation of substantial coal deposits that we extract today. Contemporary evolutionary theory asserts that all plants, encompassing specific green algae and land plants, share a common ancestry, forming a monophyletic group. The transition from aquatic to terrestrial life posed significant challenges for plants. To combat desiccation, disperse reproductive cells through the air, provide structural support, and effectively capture sunlight, plants had to develop innovative strategies. While adaptations have enabled seed plants to thrive in even the driest conditions on Earth, not all plants have attained complete independence from water. Although many seedless plants still require moisture for reproduction, their adaptations have allowed them to survive and flourish in diverse environments.
UNIT 2 THE SEEDLESS PLANTS
Introduction
Intended Learning Outcomes (ILOs)
Main Contents
The Major Divisions of Land Plants
The Diversity of the Green Algae
Ecological Adaptation of Algae
Summary
Self-Assessment Exercises
Introduction
Vascular plants are small and seedless, and the gametophyte stage of their life cycle dominates them. They don't have roots or a circulatory system, so they absorb nutrients and water from all of their exposed surfaces. The three main groups, collectively referred to as bryophytes, are liverworts, hornworts, and mosses. The oldest plants are liverworts, which were among the first plants to appear on land. Hornworts have developed stomata and one chloroplast per cell. Mosses have simple conductive cells and are connected to the substrate by rhizoids. After drying out, they may rehydrate and colonize hostile settings. Spore discharge from the parent plant is made possible by the complex structure of the moss sporangium.
By the end of this section, you will be able to do the following:
· Describe the traits shared by green algae and land plants
· Explain why charophytes are considered the closest algal relative to land plants
· Explain how current phylogenetic relationships are reshaped by comparative analysis of DNA sequences
· Discuss the ecological adaptations of the algae
The Major Divisions of Land Plants
Streptophytes collectively refer to the green algae and land plants encompassing the Streptophyta subphylum. Within the realm of land plants, a fundamental division is based on the presence or absence of vascular tissue, as depicted in the diagram. Non-vascular plants, lacking specialized cells for water and nutrient transport, include liverworts, mosses, and hornworts. These seedless plants are likely among the earliest terrestrial plants. Vascular plants have evolved a network of cells to facilitate solute and water movement. The initial vascular plants likely resembled lycophytes such as club mosses, emerging in the late Ordovician period (500 to 435 MYA), along with pterophytes like ferns, horsetails, and whisk ferns. As they lack seed formation, lycophytes and pterophytes are termed "seedless vascular plants." However, seed plants, known as spermatophytes, dominate the plant kingdom and the landscape. Among them, gymnosperms, particularly conifers with their exposed seeds, and angiosperms, the flowering plants with the highest reproductive success, are prominent examples. Angiosperms encase their seeds within protective chambers at the flower's core, with these chambers eventually developing into fruits. The diversity of Embryopyhtes can be shown in the figure below:
Figure 2.1 Major divisions of green plants (Streptophytes). Source: Biology 2e, Biological Diversity, Seedless Plants, Early Plant Life | OpenEd CUNY
Characteristic Forms of Algae
Plants and animals share specific general properties of algae. Eukaryotic cells make up algae. Algae, for example, may photosynthesize like plants and have specialized cell organelles like centrioles and flagella that are exclusively found in animals. Mannans, cellulose, and Galatians make up the algal cell walls. Listed below are some of the general characteristics of algae.
Ø Algae are photosynthetic organisms
Ø Algae can be either unicellular or multicellular organisms
Ø Algae lack a well-defined body, so, structures like roots, stems or leaves are absent
Ø Algae are found where there is adequate moisture, with a few on damp soils and shady places examples are spirogyra, anabaena and Sargassum
Ø Reproduction in algae occurs in both asexual and sexual forms.
Ø Asexual reproduction occurs by spore formation.
Ø Algae are free-living, although some can form a symbiotic relationship with other organisms.
Ø The cell wall of algae is composed of a true celloulose.
Ø Reserve carbohydrates are usually starch and not glycogen as in fungi.
Classification of Algae
Grouping creatures based on the traits they simulate is known as classification. From an evolutionary perspective, it is not improbable but true that animals with comparable morphology, life cycles, physiologies, and biochemistry are genetically related. Therefore, nine large taxonomic groups known as Divisions are recognized in the categorization of algae. These are:
Chlorophycophyta (green algae) eg. Chlamydomonas, Spirogyra, Chlorella
Xanthophycophyta (yellow-green algae) eg. Vaucheria, Botrydium
Bacillariophycophyta (diatoms) eg. Diatoma, fragilaria.
Phaeophycophyta (brown algae) eg. Fucus, Sargassum, laminaria.
Rhodophycophyta (red algae) eg. Plumaria elegans.
Chrysophycophyta (golden algae) eg. Synura, Mallomonas, Chromalina.
Euglenophycophyta (euglenoids) eg. Euglena, Trachelommas.
What is the striking difference between algae and protists?
Self-Assessment Exercises 1
1. What is the meaning of land plant?
2. What is another name for land plants?
2.3.2 The Diversity of the Green Algae
Algae, referred to as alga in singular form, constitute a diverse group of primarily aquatic photosynthetic organisms within the Protista kingdom. Algae exhibit a wide range of life cycles and vary in size from tiny Micromonas species to enormous kelps that can extend up to 60 meters (200 feet). These organisms possess a greater variety of photosynthetic pigments compared to plants and display unique cellular characteristics distinct from both plants and animals. Besides their essential roles in generating oxygen and serving as a fundamental food source for aquatic life, algae hold economic significance as sources of crude oil, food, pharmaceuticals, and various industrial products for humans. The classification of algae is subject to ongoing debates and swift changes as new molecular insights emerge. The field dedicated to studying algae is termed phycology, and individuals who specialize in this area are known as phycologists. Evolutionarily, algae do not share close relationships, and the exact evolutionary lineage of the group remains to be fully elucidated. Certain algae share attributes with protozoa and fungi, which makes differentiation from these organisms challenging when chloroplasts and photosynthesis are not distinct defining features. In some cases, certain algae seem to share a closer evolutionary connection with protozoa or fungi than with other algae. In this context, we will explore the two primary categories of green algae.
1 Streptophytes
Until recently, all eukaryotic organisms capable of photosynthesis were categorized within the Plantae kingdom. Nevertheless, the brown and golden algae have been recently reclassified into the Chromalveolata supergroup of protists. This reclassification is due to their divergence from plants in terms of structural and biochemical characteristics, despite sharing the capacity to harness light energy and fix CO2. The current classification designates plants, along with red and green algae, under the Archaeplastida supergroup of protists. Green algae exhibit similar carotenoids, chlorophyll a and b, and starch storage as land plants. Their cells possess chloroplasts with diverse shapes, and their cell walls contain cellulose, mirroring features found in land plants. The exact inclusion of specific green algae within the plant category remains unresolved from a phylogenetic perspective.
Green algae can be broadly divided into two main groups: chlorophytes and charophytes. Chlorophytes encompass genera like Chlorella, Chlamydomonas, the seaweed "sea lettuce" Ulva, and the colonial alga Volvox. Charophytes encompass desmids, along with genera like Spirogyra, Coleochaete, and Chara. Notably, recognizable green algae exist within both groups. Some green algae, such as Chlamydomonas and desmids, consist of single cells, adding complexity to their classification due to the multicellular nature of plants. Conversely, other green algae, like Volvox, form colonies, while organisms like Ulva are multicellular entities. Spirogyra, characterized by long filaments of colonial cells, primarily inhabit freshwater, brackish water, seawater, or even snowy patches. A few green algae can even survive on soil, provided a thin layer of moisture covers it, allowing them to endure dry periods
Figure 2.2 Green algae. Charophyta include (a) Spirogyra and (b) desmids. Chlorophyta include (c) Chlamydomonas, and (d) Ulva. Desmids and Chlamydomonas are single-celled organisms, Spirogyra forms chains of cells, and Ulva forms multicellular structures resembling leaves, although the cells are not differentiated as they are in higher plants. Source: https://openstax.org/books/conceptsbiology/pages/14-1-the-plant-kingdom
Differences between chlorophytes and charophytes provide insight, along with molecular analysis, into the relationship between land plants and charophytes. Charophytes and land plants share specific characteristics that place them as sister groups. Firstly, cell division in charophytes and land plants occurs through phragmoplasts, where microtubules parallel to the spindle guide forming cell plates. On the other hand, chlorophytes use phycoplasts with perpendicular microtubules for cell plate organization. Secondly, only charophytes and land plants possess plasmodesmata, facilitating material transfer between cells. Such intercellular connections do not persist in mature multicellular chlorophyte forms. Lastly, both charophytes and land plants exhibit apical growth—growth from plant tips instead of throughout the plant body. Consequently, land plants and charophytes constitute a newly identified monophyletic group called Streptophyta.
Green algae undergo both asexual and sexual reproduction. Asexual methods involve spore dispersal or fragmentation, while sexual reproduction entails the fusion of gametes. In single-celled organisms like Chlamydomonas, fertilization lacks subsequent mitosis. In the multicellular Ulva, a sporophyte develops through mitosis after fertilization, showcasing alternation of generations. Both Chlamydomonas and Ulva produce flagellated gametes.
2. Charophytes
Among the charophytes, several distinct algal orders have been proposed as the closest relatives of land plants: the Charales, Zygnematales, and Coleochaetales. The Charales, with a history dating back 420 million years, inhabit diverse freshwater environments and range from a few millimeters to a meter in length. The prominent genus Chara, also known as muskgrass or skunkweed due to its unpleasant odor, represents this group. The thallus, or main stem, consists of large cells, while branches emerging from nodes consist of smaller cells. Reproductive structures of both sexes are located on nodes, with flagellated sperm. Despite superficial resemblances to some land plants, a notable distinction is the absence of supportive tissue in the stem of Chara. However, Charales possess key traits relevant to adapting to terrestrial life, including the production of lignin and sporopollenin compounds, along with plasmodesmata connecting adjacent cells. Despite their haplontic life cycle (with the main form being haploid and short-lived diploid zygotes formed), eggs and later zygotes develop within a sheltered chamber on the haploid parent plant.
Figure 2.3 Chara. The representative alga, Chara, is a noxious weed in Florida, where it clogs waterways. Source: https://openstax.org/books/concepts-biology/pages/14-1-the-plantkingdom
The multicellular forms known as coleochaetes are branching or discshaped. Although they are capable of both sexual and asexual reproduction, their life cycle is essentially haplontic. The Zygnematales are more closely connected to the embryophytes than the Charales or the Cof charophytes. The desmids and the well-known genus Spirogyra both belong to the Zygnematales. The study of the evolutionary relationships between charophytes and land plants will continue as DNA analysis methods advance and new knowledge on comparative genomics emerges in order to provide an acceptable answer to the question of where land plants came from. How do algae produces sexually?
Self-Assessment Exercises
1 What are the Streptophytes? 2. What are the two major groups of the green algae?
Ecological Adaptation of Algae
Aquatic algae are one of the simplest plant organisms on our planet. If life evolved from bacteria to plants, from sea creatures to land creatures, algae is likely one of the primal stepping stones in the evolutionary process. Aquatic algae demonstrate properties which are found in animal life and plant life, including the ability to adapt to its surroundings.
1. Interaction with Competition Like chameleon lizards that change colour to blend in with their surroundings, individual algae strains have demonstrated the ability to uniquely adapt to individual environments in order to blend in with, or overcome the challenges of, their environment. In a coral reef setting, algae become highly regenerative.
2. Reproductive Adaptation Asexual reproduction allows a plant or animal species to procreate independently and endure in a hostile environment. A male and a female of the species are required for sexual reproduction in order to supply all of the components for a full regeneration phase. Mammals, for instance, need a male sperm to fertilize a female egg in order to produce a new life. Algae that live in water have evolved the capacity for both asexual and sexual reproduction. Fragmentation is the method used for asexual reproduction. The plant releases spores that have the ability to germinate and thrive. When gametes from two species come together to form a spore known as a syngamy, sexual reproduction takes place. This spore then develops and releases algal seeds, mature cells that can grow or reproduce. Asexual reproduction enables a plant or animal species to reproduce on its own and survive in a highly competitive surrounding.oleochaetales, according to a recent in-depth DNA sequence analysis
3. Environmental Adaptation Aquatic algae demonstrate photosensitive and cosmetic adaptation throughout the ocean. If algae originated from a single strain, environmental evolutions have forced the adaptation into red, brown, yellow and green colour algae which each blend in with their environmental surroundings. This adaptation helps algae avoid being completely consumed by the local fish species.
4. Internal Chemical Adaptation In a research experiment conducted in the San Francisco Bay area, multiple samples of several algae species were taken from polluted and non-polluted waters. Tests demonstrated that the algae residing in polluted waters had internally adapted, and produced chemical variances as part of the adaptation process. These plants physically and chemically changed their composition in order to adapt to the pollutants in the water.
Self-Assessment Exercises 3
1. Are algae plants or bacteria?
2. What is the purpose of algae?
Summary
Charophytes share more traits with land plants than do other algae, according to structural features and DNA analysis. Within the charophytes, the Charales, the Coleochaetales, and the Zygnematales have been each considered as sharing the closest common ancestry with the land plants. Charophytes form sporopollenin and precursors of lignin, phragmoplasts, and have flagellated sperm. They do not exhibit alternation of generations.
UNIT 3 BRYOPHYTES
3.1 Introduction
3.2 Intended Learning Outcomes (ILOs)
3.3 main Contents
3.3.1 The Bryophytes
3.3.2 Diversity of Bryopyta
3.3.2.1 Hepaticopsida (Liverworts)
3.3.2.2 Anthocerotopsida (Hornworts)
3.3.2.3 Bryopsida (Mosses)
3.3.3 Adaptation of Bryophytes
3.3.3.1Habitat of Bryophyte
3.3.3.2Comparison of gametophytic and sporophytic phases of bryophytes
3.4 Summary
3.5 Self-Assessment Exercises
3.1 Introduction
During the course of evolution, a change from aquatic habitat to terrestrial habitat occurred and the only primitive land plants evolved. These are known as bryophytes. Although bryophytes colonize terrestrial habitats but they are still dependent on water for completion of their life cycle. They produce motile male gametes which require a thin film of water for their motility to reach the non-motile female gamete to accomplish fertilization.
3.2 Intended Learning Objectives By the end of this section, you will be able to:
1. Identify the main characteristics of bryophytes
2. Describe the distinguishing traits of liverworts, hornworts, and mosses
3. Chart the development of land adaptations in the bryophytes
4. Describe the events in the bryophyte lifecycle
5. Describe the major classes of seedless vascular plants
3.3.1 The Bryophytes
The earliest terrestrial plants, which are most closely related to bryophytes, likely emerged around 450 million years ago during the Ordovician epoch. Bryophytes are believed to have originated during this time, but their lack of durable components like lignin makes them unlikely to leave fossils. Some spores protected by early bryophytes have managed to survive. By the Silurian epoch (around 435 million years ago), vascular plants had already spread across all continents. This supports the idea that non-vascular plants existed before the Silurian epoch. Although over 25,000 types of bryophytes flourish mainly in moist environments, some also survive in deserts. In challenging environments like the tundra, where their small size and resistance to drying out provide significant benefits, they dominate the flora. Bryophytes are often called non-vascular plants, even though the term non-tracheophyte is more precise. Key vegetative structures of bryophytes, such as photosynthetic leaf-like parts, stems, rhizoids, and thalli (the "plant body"), belong to the haploid organism, or gametophyte. Bryophyte male gametes possess a flagellum for swimming, necessitating water for fertilization. Moreover, the bryophyte embryo remains attached to the parent plant, which nurtures and nourishes it. The sporophyte that emerges from the embryo is not very visible.
The multicellular sexual reproductive structure called the sporangium, responsible for producing haploid spores through meiosis, exists in bryophytes and is absent in most algae. This feature is also shared among land plants. Why are plants called bryophytes?
The general characteristics of Bryophytes can be outlined as follows:
1. Bryophytes are amphibians of plant kingdom as they complete their life cycle in both water and on land.
2. Plants occur in damp and shaded areas.
3. The plant body is thallus-like, ie. prostrate or erect.
4. It is attached to the substratum by rhizoids, which are unicellular or multicellular.
5. They have a root-like, stem-like, and leaf-like structure and lack true vegetative structure.
6. Plants lack the vascular system (xylem, phloem).
7. The dominant part of the plant body is the gametophyte which is a haploid.
8. The thalloid gametophyte is divided into rhizoids, axis, and leaves.
9. The gametophyte bears multicellular sex organs and is photosynthetic.
10. The antheridium produces antherozoids, which are flagellated.
11. The shape of an archegonium is a sort of a flask and produces one egg.
12. The antherozoids fuse with an egg to make a zygote.
13. The zygote develops into a multicellular sporophyte.
14. The sporophyte is semi-parasitic and dependent on the gametophyte for its nutrition.
15. Cells of sporophyte undergo meiosis to form haploid gametes which form a gametophyte
16. The juvenile gametophyte is known as protonema.
17. The sporophyte is differentiated into foot seta and capsule.
Self-Assessment Exercises 1
1. Which plants are bryophytes?
1. Diversity of Bryopytes
The bryophytes are divided into three divisions (in plants, the taxonomic level “division” is used instead of phylum):
1. Hepaticopsida (Liverworts): Flat, ribbon-like – Liverworts (Marchantia).
2. Anthocerotopsida (Hornworts): Flat, thalloid plant body bearing a horn-like sporophyte – Hornworts or Anthoceros
3. Bryopsida (Mosses): Small, leafy plant body – Mosses (Funaria)
3.3.2.1Hepaticopsida (Liverworts) Liverworts (Marchantiophyta) are considered the plants most closely resembling the ancestor that transitioned to land. They have successfully established themselves in diverse habitats worldwide, with over 6,000 extant species. Certain gametophytes take on a lobate green structure resembling the lobes of the liver, which is the origin of their common name. Liverworts belong to the class Hepaticopsida, and they are a subset of bryophytes, with around 900 species. Liverworts are among the most primitive bryophytes and are often found in damp environments like moist rocks and wet soil, reducing their risk of desiccation due to their proximity to water.
Gametophytes, a plant type, can exhibit a flat thalloid or ribbon-like morphology, often showing dichotomous branching. For instance, Marchantia is anchored to the soil by rhizoids. On the other hand, species like Porella grow upright and appear leafy, with differentiated fake stems and leaves. The gametophyte provides nourishment and shelter for the sporophyte. The reproductive organs are typically located at the tips of branches on the upper surface of the thallus. In cases like Marchantia, separate branches called antheridiophores and archegoniophores may develop on gametophytes to host sex organs.
Figure 3.1 (a) A 1904 drawing of liverworts shows the variety of their forms. (b) A liverwort, Lunularia cruciata, displays its lobate, flat thallus. The organism in the photograph is in the gametophyte stage. Source: https://openstax.org/books/concepts-biology/pages/14-1-theplant-kingdom
3.3.2.2Anthocerotopsida (Hornworts)
Hornworts (Anthocerotophyta) have successfully inhabited a range of terrestrial environments, although they always remain in close proximity to a moisture source. Approximately 100 hornwort species have been identified. They fall under a single order, Anthocerotales, and include genera like Anthoceros, Megaceros, and Notothylas. In comparison to Bryopsida and Hepaticopsida, this category of bryophytes displays a slightly more advanced nature in various aspects.
On a broader scale, the gametophyte of hornworts is characterized by pronounced lobes and irregularities. Unlike the early developmental stages, the sporophyte does not rely on the gametophyte for sustenance or protection. Antheridia and archegonia are partially submerged within the gametophytic tissue. The most prominent phase of the hornwort life cycle is the brief blue-green gametophyte stage. The distinctive feature of the group is the sporophyte, a slender, pipe-like structure that emerges from the parental gametophyte and continues to grow throughout the plant's life (Figure 3.2).
Figure 3.2 Hornworts grow a tall and slender sporophyte. (credit: modification of work by Jason Hollinger) Source: https://openstax.org/books/concepts-biology/pages/14-1-the-plantkingdom
3.3.2.3 Bryopsida (Mosses)
With about 1400 species, it is a significant class of the Bryophyta. They are often referred to as mosses. Like liverworts, most mosses like moist surroundings. Unlike other bryophytes, they thrive in moderately dry settings. While mosses require water to proliferate, which is why they typically develop into cushions or mats. Examples include Sphagnum, Funaria, and Polytrichum. Their habitats range from the understory of tropical forests to the tundra, where they serve as the primary vegetation. Their short rhizoids enable them to attach to a substrate in the tundra without digging into the frozen ground. They prevent erosion, hold onto moisture and soil minerals, offer food for larger herbivores like the musk ox and shelter for lesser animals. Mosses are very sensitive to air pollution and are used to monitor the quality of air. The sensitivity of mosses to copper salts makes these salts a common ingredient of compounds marketed to eliminate mosses in lawns (Figure 3.3). Are bryophytes unicellular or multicellular?
Figure 3.3 This green feathery moss has reddish-brown sporophytes growing upward. Source: https://openstax.org/books/concepts-biology/pages/14-1-theplant-kingdom
Self-Assessment Exercises 2
1. What is the classification of bryophytes?
2. What are the main groups of bryophytes?
3.3.3 Adaptation of Bryophytes
Bryophytes possess remarkable resilience and unique abilities to survive in harsh environmental conditions. They demonstrate high phenotypic plasticity and the capacity to undergo photosynthesis when conditions are favorable, particularly enabling their survival in cold regions. Bryophytes can endure drought by entering a metabolic shutdown and later recovering under suitable conditions. Anchored to soil by rhizoids, small thread-like structures, they absorb water and nutrients. In mosses, rhizoids are multicellular and branched, while liverworts and hornworts have unicellular or multicellular rhizoids. Water distribution is facilitated by hydroids and leptoids in various bryophytes, and mosses possess an external capillary system for this purpose.
The dominant gametophyte phase of bryophytes is photosynthetically active and bears the sex organs, with antheridia as male and archegonia as female. After fusion, gametes develop into a zygote and then a sporophyte, which remains attached to the gametophyte and relies on it for nutrients. The sporophyte's sporogenous tissue undergoes meiosis to produce haploid spores. Upon dispersal, these spores germinate into new gametophytes, completing the alternation of generations life cycle seen in all three bryophyte types (mosses, liverworts, and hornworts).
3.3.3.1Habitat of Bryophyte
Bryophytes—the group of seedless plants containing mosses, hornworts, and liverworts—often live in wetter habitats. They do not have to search far for water and nutrients, so they do not need very complex roots. Water loss does not present a large problem, so they do not need a waxy surface to retain water.
3.3.3.2Comparison of gametophytic and sporophytic phases of bryophytes
Self-Assessment Exercises 3
1. What are the adaptations of bryophytes?
2. What is the habitat of bryophytes?
Summary
A category of plant species known as bryophytes reproduce by spores rather than flowers or seeds. The three non-vascular land plant kinds that make up the majority of bryophytes are mosses, hornworts, and liverworts. They are typically found in moist conditions. While any of the several plants with specialized vascular tissue is considered a vascular plant. The movement of water, nutrients, and the byproducts of photosynthesis throughout the plant is accomplished by the two forms of vascular tissue, xylem and phloem.
UNIT 4 SEEDLESS VASCULAR PLANTS
Unit Structure
4.1 Introduction
4.2 Intended Learning Outcomes (ILOs)
4.3.1 Seedless Vascular Plants
4.3.1.1 Diversity of Seedless Vascular Plant Groups
4.3.1.2 General Characteristics
4.3.2 The Life Cycle of Seedless Vascular Plants
4.3.2.1 Fern life cycle
4.3.2.2 Homospory versus heterospory
4.3.3 Ecological Adaptation
4.3.3.1 Relationship of Seedless with Bryophytes
4.3.3.2 The Importance of Seedless Plants
4.4 Summary
4.5 Self-Assessment Exercises
4.1 Introduction
The vascular plants are the dominant and most conspicuous group of land plants. There are about 275,000 species of vascular plants, which represent more than 90 percent of Earth’s vegetation. Several evolutionary innovations explain their success and their spread to so many habitats.
4.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to do the following:
1. Identify the new traits that first appear in seedless tracheophytes
2. Discuss how each trait is important for adaptation to life on land
3. Identify the classes of seedless tracheophytes
4. Describe the life cycle of a fern
5. Explain the role of seedless plants in the ecosystem
4.3.1 Seedless Vascular Plants
Vascular plants, also known as tracheophytes, constitute the prominent group of land plants, accounting for over 260,000 species, which form more than 90 percent of terrestrial vegetation. Their success in occupying diverse habitats can be attributed to evolutionary innovations. While bryophytes were capable of transitioning from water to land, their reliance on water for reproduction and limited ability to absorb nutrients through the gametophyte surface restricted their size and habitat range. Vascular plants, in contrast, have developed roots for water and mineral absorption from the soil, along with specialized conducting tissues that facilitate the transport of water, minerals, and nutrients throughout the plant's structure. This adaptation enables vascular plants to attain significant heights, effectively competing for sunlight. Across the course of plant evolution, the sporophyte generation's dominance has progressively increased. In seedless vascular plants, such as ferns, the diploid sporophyte phase takes precedence in the life cycle, while the gametophyte, although less conspicuous, remains independent of the sporophyte. However, water dependence persists during fertilization in seedless vascular plants, as flagellated sperm require moisture to reach the egg. Consequently, ferns and related species thrive primarily in moist environments due to this reproductive limitation.
1. Vascular Tissue: Xylem and Phloem
The Silurian period, which began roughly 430 million years ago, is when the earliest fossils with evidence of vascular tissue were discovered. The simplest configuration of conductive cells displays a pattern with xylem at the center and phloem on either side. The tissue known as xylem is in charge of carrying water and minerals over long distances, moving water-soluble growth factors from organs of synthesis to target organs, and storing water and nutrients.
Phloem, a different kind of vascular tissue, moves carbohydrates, proteins, and other solutes throughout the plant. Sieve components, also known as conducting cells, and supporting tissue are two categories of phloem cells. The vascular system of plants is composed of both xylem and phloem components.
2. Roots: Support for the Plant
Roots, although not well-preserved in the fossil record, seem to have emerged later in evolution compared to vascular tissue. The development of an intricate root system marked a significant advancement in vascular plants. Unlike the delicate filaments called rhizoids in bryophytes, which only loosely anchored the plant and didn't absorb water and nutrients effectively, roots with their robust vascular tissue system serve as a conduit for transporting water and minerals from the soil throughout the plant. This extensive network of roots not only ensures access to water sources deep in the ground but also provides stability to trees, acting as both ballast and an anchor.
Most roots establish a mutually beneficial relationship with fungi, forming mycorrhizae. In this symbiotic interaction, fungal hyphae grow around the root, penetrating the root cells or even residing within them. This relationship significantly enhances the plant's absorption capabilities by greatly increasing the surface area available for nutrient uptake.
3. Leaves, Sporophylls, and Strobili
A third adaptation marks seedless vascular plants. Accompanying the prominence of the sporophyte and the development of vascular tissue, the appearance of true leaves improved photosynthetic efficiency. Leaves capture more sunlight with their increased surface area. In addition to photosynthesis, leaves play another role in the life of the plants. Pinecones, mature fronds of ferns, and flowers are all sporophylls—leaves that were modified structurally to bear sporangia. Strobili are structures that contain the sporangia. They are prominent in conifers and are known commonly as cones: for example, the pine cones of pine trees. In the evolution andadaptation of seedless vascular plants what does the appearance of true leaves signifies?
4.3.1.1 Diversity of Seedless Vascular Plant Groups
Seedless vascular plants are mainly split into two groups, the lycophytes and the monilophytes. These aren’t common names, however, and might be a little confusing to remember. Below we go over what each of these names means and some examples of seedless vascular plants.
1. The Lycophytes
This group includes quillworts, spike mosses, and club mosses. Despite the term "moss" in their names, they are not true nonvascular mosses, as they possess vascular systems. Lycophytes differ from monilophytes in their leaf-like structures known as "microphylls," which have a single vein of vascular tissue and lack branching. Lycophytes exhibit alternation of generations similar to bryophytes, but with the sporophyte as the predominant life stage. Gametophytes in lycophytes are independent of the sporophyte for nutrients and may form mycorrhizal associations. In club mosses, the sporophyte develops sporophylls arranged in strobili, cone-like structures. Lycophytes can be either homosporous or heterosporous.
An example within this group is the club moss. These plants, dominant during the Carboniferous period, were sizable trees forming extensive swamp forests. Present-day club mosses are small evergreen plants with stems and microphylls. The Lycophyta division comprises nearly 1,000 species, encompassing quillworts, club mosses, and spike mosses. These plants produce spores within cone-like strobili for the development of haploid gametophytes, except for quillworts and silver mosses which bear spores on their microphylls.
Figure 4.1 Lycopodium clavatum is a club moss. Source: https://openstax.org/books/concepts-biology/pages/14-1-the-plantkingdom
2. The Monilophytes
The "euphylls" or real leaves, the plant parts we today specifically think of as leaves, distinguish the monilophytes from the lycophytes. These "euphylls" are wide and lacerated with numerous veins. The ferns and horsetails are two of the plants in this class that you may be familiar with by their popular names. Ferns contain large leaves and sori, which are spore-bearing structures, under their leaves.
i. Horsetails
Horsetails possess "euphylls," which are actual leaves that have been minimized, resulting in their slender and non-wide structure, unlike the broader leaves of ferns. These horsetail leaves are positioned in a circular arrangement, known as a "whorl," along the stem. Nevertheless, the shared characteristic connecting club mosses, spike mosses, quillworts, ferns, and horsetails is their existence prior to the development of seeds. Instead of seeds, these lineages propagate their gametophyte phase using spores. Ferns and whisk ferns are categorized within the Pterophyta division. Among the plant groups in the Pterophyta, horsetails, forming a distinct category from ferns at times, belong here. Horsetails are represented by a sole genus, Equisetum. They endure as the remnants of a once-extensive plant category referred to as Arthrophyta, which once gave rise to towering trees and entire swamp forests during the Carboniferous period. Typically favoring damp habitats and marshy environments, these plants can be found (Figure 4.2).
Figure 4.2 Horsetails thrive in a marsh. Source: https://openstax.org/books/concepts-biology/pages/14-1-the-plantkingdo
The stem of a horsetail is characterized by the presence of joints, or nodes: hence the name Arthrophyta, which means “jointed plant”. Leaves and branches come out as whorls from the evenly spaced rings. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem (Figure 4.3).
Figure 4.3 Thin leaves originating at the joints are noticeable on the horsetail plant. Source: https://openstax.org/books/conceptsbiology/pages/14-1-the-plant-kin
ii. Ferns and Whisk Ferns Fern
s are advanced seedless vascular plants that exhibit characteristics commonly found in seed plants. They possess large leaves and branching roots. In contrast, whisk ferns (psilophytes) lack roots and leaves due to evolutionary reduction, a process driven by natural selection in response to changing environments. Whisk ferns perform photosynthesis in their green stems, with sporangia-containing yellow knobs at the branch tip. While traditionally classified apart from true ferns, recent DNA analysis suggests that whisk ferns might have lost vascular tissue and roots through evolution, actually being closely related to ferns. Ferns are easily recognizable due to their sizable fronds and represent the most prominent seedless vascular plants. Approximately 12,000 fern species inhabit diverse environments from tropics to temperate forests. While some tolerate arid conditions, most ferns prefer moist and shaded habitats. Their fossil record dates back to the Devonian period (416–359 million years ago), and they flourished during the Carboniferous period (359–299 million years ago) (Figure 4.5).
Figure 4.4 Some specimens of this short tree-fern species can grow very tall. Source: https://openstax.org/books/concepts-biology/pages/14-1- the-plant-kingdom
4.3.1.2 General Characteristics
1. Pteridophytes are considered as the first plants to be evolved on land: It is speculated that life began in the oceans, and through millions of years of evolution, life slowly adapted on to dry land.
And among the first of the plants to truly live on land were the Pteridophytes.
2. They are cryptogams, seedless and vascular: Pteridophytes are seedless, and they reproduce through spores. They contain vascular tissues but lack xylem vessels and phloem companion cells.
3. The plant body has true roots, stem and leaves: They have welldifferentiated plant body into root, stem and leaves.
4. Spores develop in sporangia: The sporangium is the structures in which spores are formed. They are usually homosporous (meaning: one type of spore is produced) and are also heterosporous, (meaning: two kinds of spores are produced.)
5. Sporangia are produced in groups on sporophylls: Leaves that bear the sporangia are termed as sporophylls. The tip of the leaves tends to curl inwards to protect the vulnerable growing parts.
6. Sex organs are multicellular: The male sex organs are called antheridia, while the female sex organs are called archegonia.
7. They show true alternation of generations: The sporophyte generation and the gametophyte generation are observed in Pteridophytes. The diploid sporophyte is the main plant body.
Why are the monilophytes separated from the lycophytes?
Self-Assessment Exercises 1
1. What are the characteristics feature of the plant body of a seedless vascular plant?
2. What is the most common seedless vascular plants?
4.3.2 The Life Cycle of Seedless Vascular Plants
The seedless vascular plants go through an alternation of generations just as the nonvascular plants and other vascular plants do. The diploid sporophyte, however, is the more prevalent, noticeable generation. Both the diploid sporophyte and haploid gametophyte are independent of each other in the seedless vascular plant.
4.3.2.1Fern life cycle
The life cycle of a fern, for example, follows these steps.
1. The mature haploid gametophyte stage has both male and female sex organs- or antheridium and archegonium, respectively.
2. The antheridium and archegonium both produce sperm and eggs via mitosis, as they are already haploid.
3. The sperm must swim from the antheridium to the archegonium to fertilize the egg, meaning the fern depends on water for fertilization.
4. Once fertilization happens, the zygote will grow into the independent diploid sporophyte.
5. The diploid sporophyte has sporangia, which is where the spores are produced via meiosis.
6. On the fern, the underside of the leaves have clusters known as sori, which are groups of sporangia. The sori will release spores when they mature, and the cycle will restart.
Notice that in the fern life cycle, although the gametophyte is reduced and the sporophyte is more prevalent, the sperm still relies on water to reach the egg in the archegonium. This means that ferns and other seedless vascular plants must live in damp environments to reproduce.
4.3.2.2Homospory versus heterospory
The majority of vascular seedless plants are homosporous, meaning they only generate one type of spore, which develops into a gametophyte with both male and female sex organs. Some, however, are heterosporous, which means they produce both megaspores and microspores. Megaspores develop into a gametophyte with exclusively female sex organs. A male gametophyte with only male sex organs develops from microspores. Although heterospory is not widespread in all vascular plants that produce seeds, it is common in those that do.
Given that many plants that produce seeds have this adaptation, evolutionary biologists think that the development of heterospory in seedless vascular plants was a significant step in the evolution and diversification of plants. The sexuality of pteridophytic gametophytes can be classified as follows:
1. Dioicous: the individual gametophyte is either a male producing antheridia and sperm or a female producing archegonia and egg cells.
2. Monoicous: every individual gametophyte may produce both antheridia and archegonia and it can function both as a male as well as a female.
3. Protandrous: the antheridia matures before the archegonia.
4. Protogynous: the archegonia matures before the antheridia.
Pteridophyta is one of the older groups of plants present in the Plant kingdom. They have evolved much earlier than the angiosperms. They are one of the very first “true” plants to adapt to life on land.
Self-Assessment Exercises 2
1 What is Homospory?
2. Name the male and female reproductive organs in pteridophytes?
4.3.3 Ecological Adaptation
Seedless vascular plants are early vascular plants that contain a number of adaptations that helped them survive life on land. You will notice that a lot of the characteristics that developed in the seedless vascular plants are not shared with nonvascular plants.
1. Vascular tissue
This is a novel adaptation. The development of the tracheid, a type of elongated cell that makes up the xylem, in early land plants led to the adaptation of vascular tissue. Xylem tissue contains tracheid cells fortified by lignin, a strong protein, that provides support and structure to vascular plants. The vascular tissue includes the xylem, which transports water, and the phloem, which transports sugars from the source (where they are made) to sink (where they are used).
2. True roots, stems, and leaves
The introduction of the vascular system in lineages of seedless vascular plants marked a significant advancement, leading to the emergence of genuine roots, stems, and leaves. This transformation revolutionized how plants engaged with their environment, enabling them to achieve larger sizes and inhabit new terrains. Following the establishment of vascular tissue, true roots emerged, capable of delving deeper into the soil for stability and nutrient absorption. These roots often form mycorrhizal associations, connecting with fungi to trade sugars for soilextracted nutrients. Through mycorrhizae and expansive root systems, vascular plants enhance their soil interface, facilitating rapid water and nutrient uptake. The vascular tissue enabled water transport from roots to stems and leaves, essential for photosynthesis. Moreover, it facilitated the distribution of sugars generated in photosynthesis to non-photosynthetic parts. The development of vascular stems centralizes the plant body, enabling substantial growth. Microphylls, found in lycophytes like club mosses, represent the earliest leaf-like structures in vascular plants, characterized by a single vein of vascular tissue. On the other hand, euphylls, found in ferns, horsetails, and other vascular plants, are genuine leaves with multiple veins and interveinal photosynthetic tissue.
3. A dominant sporophyte generation
Unlike the nonvascular plants, the early vascular plants developed a dominant diploid sporophyte generation, independent of the haploid gametophyte. Seedless vascular plants also have a haploid gametophyte generation, but it is independent and reduced in size compared to nonvascular plants.
4.3.3.1Relationship of Seedless with Bryophytes
a). Similarities
1. Both are land plants.
2. Both possess distinct Gametophytic and Sporophytic generations.
3. Both groups show heteromorphic alternation of generation.
4. Rhizoids are present in both Bryophytes and Pteridophytes.
5. Sexual reproduction is oogamous.
6. Both consist of multicellular sporangia.
7. The cuticle is present in both plants.
8. Both groups have members with terrestrial mode of life.
9. Formation of spores is the same in both groups.
10. Like Bryophytes, many seedless plants are homosporous.
b). Differences
Following are the major differences between bryophytes and seedless plants:
4.3.3.2 The Importance of Seedless Plants
Mosses and liverworts are pioneering organisms that often colonize areas during primary or secondary successions. Their spores are dispersed by various means. Once established, they provide nourishment and shelter for other plants. In harsh environments like frozen tundras, bryophytes thrive due to their adaptability. Mosses, foundational in tundra food chains, sustain a variety of species. Bryophytes can enhance soil for other plants by forming symbiotic relationships with nitrogenfixing cyanobacteria, replenishing soil nitrogen. Urbanization led to moss and lichen decline, indicating pollution susceptibility. Mosses are sensitive to pollutants due to their direct absorption of rainwater-borne substances. Their disappearance serves as a pollution indicator. Ferns play roles in weathering, soil formation, and erosion control. Water ferns like Azolla restore nitrogen in aquatic habitats via cyanobacteria.
Historically, seedless plants were used as tools, fuel, and medicine. Sphagnum moss, used for fuel and soil conditioning, is renewable. Ferns are popular ornamentals and houseplants. Bracken fern fiddleheads are a Native American spring delicacy. Licorice fern serves as food and medicine for Pacific Northwest tribes due to its sweetness. These seedless plants have intertwined with human life for various purposes. What are the first leaf-like structures that evolved in vascular plants? Microphylls. These are small leaf-like structures, with only a single vein of vascular tissue running through them. Lycophytes (e.g., club mosses) have these microphylls.
Self-Assessment Exercises 3
1. What are the seedless vascular plants?
2. What is the striking difference between the non-vascular and vascular plant groups
4.4 Summary
Seedless nonvascular plants are small. The dominant stage of the life cycle is the gametophyte. Without a vascular system and roots, they absorb water and nutrients through all of their exposed surfaces. There are three main groups: the liverworts, the hornworts, and the mosses.
They are collectively known as bryophytes. Vascular systems consist of xylem tissue, which transports water and minerals, and phloem tissue, which transports sugars and proteins. With the vascular system, there appeared leaves—large photosynthetic organs—and roots to absorb water from the ground. The seedless vascular plants include club mosses, which are the most primitive; whisk ferns, which lost leaves and roots by reductive evolution; horsetails, and ferns.
UNIT 5 SEED PLANT I: GYMNOSPERMS
Unit Structure
5.1 Introduction
5.2 Intended Learning Outcomes (ILOs)
5.3.1 The Evolution of Seed Plants
5.3.2 The Gymnosperms
5.3.2.1 General Characteristics
5.3.2.2 Diversity of Gymnosperms
5.3.2.3 Life Cycle of a Conifer
5.3.2.4 Ecological Adaptation of Gymnosperms
5.4 Summary
5.5 Self-Assessment Exercises
5.1 Introduction
The first plants to colonize land were most likely closely related to modern-day mosses (bryophytes) and are thought to have appeared about 500 million years ago. They were followed by liverworts (also bryophytes) and primitive vascular plants, the pterophytes, from which modern ferns are derived. The life cycle of bryophytes and pterophytes is characterized by the alternation of generations. The completion of the life cycle requires water, as the male gametes must swim to the female gametes. The male gametophyte releases sperm, which must swim—propelled by their flagella—to reach and fertilize the female gamete or egg. After fertilization, the zygote matures and grows into a sporophyte, which in turn will form sporangia, or "spore vessels,” in which mother cells undergo meiosis and produce haploid spores. The release of spores in a suitable environment will lead to germination and a new generation of gametophytes.
5.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to:
1. Discuss the type of seeds produced by gymnosperms, as well as other characteristics of gymnosperms
2. List the four groups of modern-day gymnosperms and provide examples of each list out general characteristics of seed plants outline the specific characteristics features of gymnosperms
4. highlight morphological characteristic of a named gymnosperm
5. classify gymnosperms and highlight the terrestrial (ecological) of seed plants.
5.3.1 The Evolution of Seed Plants
The evolutionary progression in seed plants led to a dominant sporophyte generation, where the diploid plant is ecologically more significant. Simultaneously, gametophytes became smaller, transitioning from prominent structures to microscopic cell clusters within sporophyte tissues. Lower vascular plants are mostly homosporous, while seed plants are heterosporous, forming male microspores and female megaspores. Gametophytes in seed plants rely on sporophyte tissue for nutrients and water, differing from the free-living gametophytes in seedless vascular plants, indicating an evolutionary connection.
Two key adaptations—seeds and pollen—set seed plants apart from seedless vascular plants and enabled their land colonization. Fossils date distinct seed plants back around 350 million years, with gymnosperms emerging during the Carboniferous period. Gymnosperms transitioned from progymnosperms and dominated during the Mesozoic era, with angiosperms taking over in the Cretaceous period, becoming the most abundant plant group in terrestrial ecosystems.
The introduction of pollen and seeds freed seed plants from waterdependent reproduction and allowed them to thrive on land. Pollen, containing male gametes, is protected from desiccation and damage, aiding in long-distance gene dispersal. Seeds safeguard embryos, offer nourishment, and maintain dormancy, promoting survival in harsh conditions and ensuring optimal germination. This advancement facilitates both spatial and temporal dispersal, leading to the remarkable success of seed plants as the most prosperous and recognizable plant group. How seeds are evolved?
Self-Assessment Exercises 1
1 What is a seed plant?
2. What are 2 types of seed plants?
5.3.2 The Gymnosperms
Gymnosperms (“naked seed”) are a diverse group of seed plants and are paraphyletic. Paraphyletic groups do not include descendants of a single common ancestor. Gymnosperm characteristics include naked seeds, separate female and male gametes, pollination by wind, and tracheids, which transport water and solutes in the vascular system.
5.3.2.1 General Characteristics
The general characteristics of gymnosperms include:
1. The adult plant (sporophyte) is a tall, woody, perennial tree or shrub mostly evergreen. The stem is usually branched, but rarely unbranched as in, Cycas.
2. Leaves may be simple (as in Pinus) or compound.
3. Leaves may be dimorphic or of one kind only. Foliage leaves are large green simple or pinnately compound, needle-like and grow on dwarf shoot as in, Pinus, or directly borne on the main trunk as in Cycas. Scale leaves are brown and simple.
4. Vascular bundles in stem are arranged in a ring and show secondary growth.
5 Gymnosperms bear cones which are usually unisexual (either male or female, rarely bisexual as in Gnetum.
6. Pollen grains are haploid produced in microsporangia of the male cones. In Pinus, each pollen grain has two large sacs, called wings to help in the dispersal by wind. Pollen grains produce two male gametes.
7. Ovules are not enclosed in ovary as in Angiosperms, but are borne naked on leafy megasporophylls of female cone, so the term gynmosperms or ‘naked seeds’ for this group. Ovules are produced side by side, inside which female gamete or egg is produced. The male gamete fuses with female gamete in the ovule.
The fertilised ovule then develops into a seed (winged in case of Pinus). Examples
5.3.2.2 Diversity of Gymnosperms
About 1,000 known species make up the four main divisions of modern gymnosperms. Coniferophyta, Cycadophyta, and Ginkgophyta are not closely connected phylogenetically, but they do produce secondary cambium (cells that create the vascular system of the trunk or stem) and have comparable seed formation patterns. Because they produce true xylem tissue that includes both tracheids and vessel components, gnetophyta are regarded as being the most closely related group to angiosperms.
1. Conifers (Coniferophyta)
With the greatest variety of species, conifers constitute the dominant phylum of gymnosperms. Most are tall trees with leaves that resemble scales or needles. The needles' thin structure and waxy coating prevent excessive water loss through transpiration. Snow keeps the burden light and lessens branch breaking by easily sliding off needle-shaped leaves. The dominance of conifers at high altitudes and in cold climates can be attributed to these adaptations to cold and dry conditions. Conifers include well-known evergreen trees including yews, pines, spruces, firs, cedars, and sequoias. (Figure 5.2). A few species are deciduous and lose their leaves all at once in fall. The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains tracheids, but no vessel elements, and is referred to as “soft wood.”
Figure 5.2 Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes. Shown here are the (a) evergreen spruce, (b) sequoia, (c) juniper, and (d) a deciduous gymnosperm: the tamarack Larix laricina. Notice the yellow leaves of the tamarack. Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seedplants-gymnosper
2. Cycads
Cycads thrive in mild climates and are often mistaken for palms because of the shape of their large, compound leaves. They bear large cones, and unusually for gymnosperms, may be pollinated by beetles, rather than wind. They dominated the landscape during the age of dinosaurs in the Mesozoic era (251–65.5 million years ago). Only a hundred or so cycad species persisted to modern times. They face possible extinction, and several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens (Figure 5.3).
Figure 5.3 This Encephalartos ferox cycad exhibits large cones. Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seed-plantsgymnosperms/
3. Gingkophytes
The single surviving species of ginkgophyte is the Ginkgo biloba (Figure 14.22). Its fan-shaped leaves, unique among seed plants because they feature a dichotomous venation pattern, turn yellow in autumn and fall from the plant. For centuries, Buddhist monks cultivated Ginkgo biloba, ensuring its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are found on separate plants. Usually, only male trees are planted by gardeners because the seeds produced by the female plant have an off-putting smell of rancid butter.
Figure 5.4 This plate from the 1870 book Flora Japonica, Sectio Prima (Tafelband) depicts the leaves and fruit of Gingko biloba, Source:
4. Gnetophytes
The three distinct plant genera in the genus Gnetophytes are the angiosperms' nearest relatives. They have broad leaves, just like angiosperms. In tropical and subtropical regions, Gnetum species are primarily vines. The deserts of Namibia and Angola are home to the unique, low-growing Welwitschia species. It could survive for as long as 2000 years. In dry regions of Mexico and the southwestern United States, the genus Ephedra is present in North America (Figure 5.5). The chemical ephedrine, a strong decongestant used in medicine, is derived from the tiny, scale-like leaves of ephedra. Ephedrine is only used in prescription medications because of its similarity to amphetamines in both chemical structure and neurological effects. What are the major groups modern gymnosperms are classified into? Modern gymnosperms are classified into four major divisions namely; Coniferophyta, Cycadophyta, Ginkgophyta and gnetophytes.
Figure 5.5 Ephedra viridis, known by the common name Mormon tea.Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seedplants-gymnosperms
5.3.2.3 Life Cycle of a Conifer
Pine trees, classified as conifers, bear both male and female sporophylls on the same plant. Like all gymnosperms, pines are heterosporous, producing male microspores and female megaspores. Within the male cones or staminate cones, microsporocytes undergo meiosis to generate microspores, which later mature into pollen grains. Each pollen grain contains two cells: a generative cell that will divide into two sperm cells, and another cell that will develop into a pollen tube cell. During spring, pine trees release abundant yellow pollen carried by the wind. Some pollen grains land on female cones. Gradually, the pollen tube grows from the pollen grain, and the generative cell within the pollen divides mitotically into two sperm cells. Eventually, one sperm cell fertilizes an egg cell, merging their haploid nuclei. Female cones, also known as ovulate cones, possess two ovules per scale. In each ovule, a single megasporocyte undergoes meiosis. Only one surviving haploid cell proceeds to develop into a female multicellular gametophyte, encapsulating an egg. Upon fertilization, the zygote matures into the embryo, enclosed by a seed coat originating from parent plant tissue. Fertilization and seed development in pine trees are prolonged, sometimes taking up to two years after pollination. The resultant seed comprises three tissue generations: the seed coat derived from parent plant tissue, the female gametophyte supplying nutrients, and the embryo itself. The life cycle of a conifer is depicted in Figure 5.1.
Figure 5.1 This image shows the lifecycle of a conifer. Source:https://viva.pressbooks.pub/introbio2/chapter/8-5-seedplants-gymnosperms/
5.3.2.4 Ecological Adaptation of Gymnosperms
Gymnosperms are seed plants adapted to life on land. They have several adaptations that make survival in diverse land habitats possible. These adaptations include:
1. Retention of the megagametophyte within a protective coating to form a seed on the parent sporophyte
2. Dissemination of the microgametophyte in durable pollen
3. Production of complex root systems
4. Extensive development of secondary xylem in the stem
Gymnosperms are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. They are found in colder regions where snowfall occurs. They are not differentiated into ovary, style and stigma, and are pollinated directly by the wind
1. They produce seeds that are not enclosed by a fruit, which allows them to disperse their seeds in dry and cold environments.
2. They have a vascular system that transports water and nutrients throughout the plant and provides structural support.
3. They maintain high rates of photosynthesis at relatively low temperature.
4. Their needles (leaves) have thick warty coatings and sunken stomatas which prevent excessive loss.
The seedless vascular plants have many adaptations helping them to survive in the conditions such as dry land and to grow in a much greater amount than non-vascular plants that are ferns, bryophytes, club mosses, and horsetails.
Self-Assessment Exercises 3
1 How do gymnosperms adapt to their environment?
2 What are the characteristics of gymnosperms that help them survive?
3. What are adaptations that gymnosperms have to survive in dry environments?
Summary
Seed plants are vascular plants that produce seeds. They go by the name Spermatophyte as well. Their leaves, stems, and roots are all fully grown. The fertilized egg of a very little gametophyte, which is entirely dependent on the sporophytes, the plant forms we see around us, grows into the seeds that contain the embryo.
The persistence and broad occurrence of seed plants are due to their effective seed dissemination. The male gamete, which is carried to the egg via pollination, fertilizes it. Next, the pollen tube that takes the male gamete to the egg grows. Since water is not required throughout this process, seed plants are actually terrestrial plants. Gymnosperms and Angiosperms, the seed plants, will be the focus of this subject.
UNIT 6 SEED PLANTS II; ANGIOSPERMS
Unit Structure
6.1 Introduction
6.2 Intended Learning Outcomes (ILOs)
6.3.1 The Angiosperms
6.3.2 The Life Cycle of an Angiosperm
6.3.2.1 Flowers and Fruits as an Evolutionary Adaptation
6.3.3 Diversity of Angiosperms
6.3.3.1 Basal Angiosperms
6.3.3.2Monocots
6.3.3.3Eudicots
6.3.4 The Role of Seed Plants
6.4 Summary
Self-Assessment Exercises
6.1 Introduction
From their humble and still obscure beginning during the early Jurassic period (202–145.5 MYA), the angiosperms, or flowering plants, have successfully evolved to dominate most terrestrial ecosystems. Angiosperms include a staggering number of genera and species; with more than 260,000 species, the division is second only to insects in terms of diversification.
6.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to:
1. Explain why angiosperms are the dominant form of plant life in
most terrestrial ecosystems
2. Describe the main parts of a flower and their purposes
3. Detail the life cycle of an angiosperm
4. Discuss the two main groups of flowering plants
6.3.1 The Angiosperms
Angiosperms, a significant plant division, constitute the majority of Earth's plant life. They are vascular seed plants where fertilization of the ovule results in seed development within a enclosed, hollow ovary. Typically found within flowers, the ovary is enveloped by male or female reproductive organs, or both. These plants, also referred to as flowering plants, inhabit all habitats except extreme conditions. They exist as epiphytes (living on other plants), as rooted and floating aquatics in both freshwater and marine environments, and as terrestrial plants with varying sizes, lifespans, and forms. Their diversity includes small herbs, parasitic plants, shrubs, vines, and towering trees. Additionally, they provide essential resources like medicine and timber. Familiar plants such as peas, mangos, coconuts, wheat, and rice are categorized as angiosperms. Their seeds are consistently enclosed within mature, fertilized ovaries known as fruits.
Figure 6.1. Phylogenetic tree of land plants indicating key evolutionary adaptations to life on land. Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seed-plantsangiosperms /
While various explanations have been proposed to account for the sudden proliferation and diversity of flowering plants, none have achieved the consensus of paleobotanists—scientists studying ancient plants. Nevertheless, new comparative genomic data has illuminated the evolution of angiosperms. Contrary to deriving from gymnosperms, angiosperms are a parallel sister clade to gymnosperms, evolving independently. From their modest and still not fully understood origins in the early Jurassic era, flowering plants, or angiosperms, have evolved to become dominant in terrestrial ecosystems. With over 250,000 species, the angiosperm group (Anthophyta) ranks second only to insects in terms of diversity. Their success is attributed to two innovative reproductive structures: flowers and fruits. Flowers serve to ensure pollination and protect the ovule and developing embryo within a receptacle. Fruits, on the other hand, aid in seed dispersal and safeguard the growing seed. Variations in fruit structures, like sweet flesh, wings, parachutes, or spines, reflect different seed dispersal strategies. Modern angiosperms are commonly classified as either monocots or eudicots, based on leaf and embryo structure. Basal angiosperms like water lilies are considered more primitive as they exhibit morphological traits shared by both monocots and eudicots.
Self-Assessment Exercises
1. What is the definition of an angiosperm?
2. What is difference between gymnosperm and angiosperm?
6.3.2 The Life Cycle of an Angiosperm
The main stage of an angiosperm's life cycle is the adult, or sporophyte, phase (Fig. 6.3). Angiosperms are heterosporous, just as gymnosperms. As a result, they produce megaspores, which create an ovule containing female gametophytes, and microspores, which produce pollen grains as the male gametophytes. Male gametophytes divide within the microsporangia of the anthers through meiosis to produce haploid microspores, which then go through mitosis to produce pollen grains. One generative cell that will divide into two sperm and one other cell that will become the pollen tube cell are both present in each pollen grain
Figure 6.3. The life cycle of a typical Angiosperm: Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seed-plantsangiosperms /
In a flower lacking a megasporangium, the formation of female gametes (egg cells) would not occur. If the flower lacked a microsporangium, the formation of male gametes (sperm cells) would not take place. Within the ovule enclosed by the carpel's ovary, the megasporangium resides, protected by integument layers and the ovary wall. Meiosis in the megasporocyte within the megasporangium yields four megaspores, of which the larger one survives to develop into the female gametophyte called the embryo sac. This megaspore undergoes divisions to form an eight-cell stage, with specific cells becoming egg, synergids, antipodals, and polar nuclei.
Upon pollen grain arrival at the stigma, a pollen tube grows from it, traversing the style and entering the ovule via the micropyle. The pollen tube delivers two sperm cells to the embryo sac. A double fertilization occurs: one sperm unites with the egg to form a diploid zygote (future embryo), while the other fuses with 2n polar nuclei, creating a triploid cell that becomes endosperm, a nutrient store. The zygote develops into an embryo with a radicle (small root) and one or two cotyledons (leaflike structures). Monocots have one cotyledon, while dicots have two, differentiating the major angiosperm groups. Seed food reserves are outside the embryo, and cotyledons transport these reserves to the developing embryo. A seed consists of integument layers forming the coat, endosperm with nutrients, and a well-guarded embryo at the center. Most flowers are monoecious (bisexual), containing both stamens and carpels, while a few self-pollinate. Monoecious flowers are referred to as "perfect" since they possess both sex organs, or as hermaphroditic in botanical terms. Dioecious plants have male and female flowers on separate individuals, facilitating cross-pollination. Double fertilization is a fertilization mechanism of flowering plants (angiosperms) that involves the joining of two male gametes (sperm) with two female gametes (egg and central cell). One sperm fuses with the egg to form a zygote, which develops into the embryo. The other sperm fuses with the central cell to form a triploid primary endosperm nucleus, which develops into the endosperm. This process is unique to angiosperms.
Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent (wind or water), or an animal, such as an insect or bird. Cross-pollination increases genetic diversity in a species.
Figure 6.4 Double fertilization occurs only in angiosperms. Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seed-plantsangiosperms /
Figure 6.5 Monoecious plants have both male and female reproductive structures on the same flower or plant. In dioecious plants, males and females reproductive structures are on separate plants. Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seed-plantsangiosperms /
6.3.2.1 Flowers and Fruits as an Evolutionary Adaptation
Angiosperms, or flowering plants, produce their gametes within separate structures often found within a flower. This flower serves as a stable environment for fertilization and embryo development, sheltered from environmental fluctuations. As the second most diverse phylum after insects, flowering plants exhibit an astonishing range of sizes, shapes, colors, scents, and arrangements in their flowers. Most flowers engage in mutualistic relationships with pollinators, and the diverse flower traits often mirror the nature of these pollination agents, showcasing coevolution. Flowers are modified sporophylls, resembling organized leaves clustered around a central stalk. Despite their varying appearances, all flowers share common structures: sepals, petals, carpels, and stamens. The peduncle attaches the flower to the plant, and sepals (collectively known as the calyx) form a base layer around the unopened bud. Sepals, usually photosynthetic, sometimes differ, as seen in lilies and tulips where petals and sepals seem identical. Petals, forming the corolla, are situated within the sepal whorl and often display vibrant hues to attract pollinators. Wind-pollinated flowers tend to be smaller, feathery, and less visually conspicuous. Sepals and petals collectively constitute the perianth. The central sexual organs—carpels and stamens—are located at the core of the flower.
Figure 6.6. Diagram of a typical flower containing both male and female reproductive tissues. Source:https://viva.pressbooks.pub/introbio2/chapter/8-5-seedplants- angiosperms
The female reproductive organ in a flower is the gynoecium or carpel, consisting of styles, stigmas, and ovules. Flowers vary greatly in structure, with carpels existing as singular, multiple, or fused units. When multiple carpels are fused, they create a pistil. Carpel tissues offer protection to the megaspores and female gametophytes. A style, a lengthy and slender structure, extends from the sticky stigma (where pollen is received) to the ovary enclosed within the carpel. The ovary contains one or more ovules, which develop into seeds after fertilization. Surrounding the central carpel are the male reproductive organs, collectively known as the androecium or stamens. Stamens comprise a thin stalk called a filament and a sac-like structure known as the anther. The filament supports the anther, within which microspores are produced through meiosis, eventually developing into pollen grains.
After fertilization of the egg, the ovule transforms into a seed. The tissues surrounding the ovary thicken, developing into a fruit that safeguards the seed and facilitates its dispersal over wider areas. Not all fruits originate from ovaries; some structures termed "false fruits." Fruit appearance, size, smell, and taste can vary greatly, with examples including tomatoes, walnut shells, and avocados. Fruits, similar to pollen and seeds, play roles in dispersal. Wind can carry certain fruits away, while others attract animals that eat the fruit, subsequently transporting the seeds through their digestive systems and depositing them elsewhere. Cockleburs, equipped with hooked spines, cling to fur or clothing and can travel long distances. These inspired the invention of Velcro by George de Mestral, who encountered them on his hike. As the seed develops, the ovary's walls thicken, forming the fruit. In botanical terms, a ripened ovary containing seeds becomes a fruit. Some foods called vegetables are technically fruit, like eggplants, zucchinis, and bell peppers, as they contain seeds and arise from thick ovary tissue. Acorns are nuts, and winged maple whirligigs (samara) also qualify as fruit. Botanists classify fruit into numerous categories, with only a few being fleshy and sweet.
Figure 6.7. Fruits develop from the ovary which surrounds the seed. Some fruits also encase the pericarp and receptacle of the floral structure.Source: https://viva.pressbooks.pub/introbio2/chapter/8-5- seed-plants- angiosperms /
Mature fruits can be categorized as fleshy or dry. Fleshy fruits encompass well-known examples like berries, peaches, apples, grapes, and tomatoes. On the other hand, dry fruits include rice, wheat, and nuts. Another distinction lies in the fact that not all fruits originate from ovaries; for instance, strawberries develop from the receptacle, and apples from the pericarp or hypanthium. Some fruits arise from multiple ovaries within a single flower, such as raspberries, while others, like pineapples, emerge from clusters of flowers. Certain fruits, like watermelons and oranges, possess rinds.
Despite their origins, fruits play a crucial role in seed dispersal. Their diverse shapes and characteristics are adapted for various modes of dispersal. Wind carries lightweight, dry fruits like those of trees and dandelions, while water transports floating coconuts. Some fruits attract herbivores through color, fragrance, or as food. Once ingested, undigested seeds pass through the herbivore's feces, contributing to dispersal. Other fruits have mechanisms like burs and hooks that cling to fur, allowing them to hitch rides on animals.
6.3.3 Diversity of Angiosperms
The Anthophyta is the only phylum in which angiosperms are categorized. It appears that modern angiosperms are a monophyletic group, which denotes that they have a single common ancestor. According to the structure of the cotyledons, pollen grains, and other components, flowering plants can be categorized into two main groupings. Grass and lilies are examples of monocots, whereas eudicots or dicots make up a polyphyletic group (Fig. 6.2). Because they have characteristics from both groups, basal angiosperms are a class of plants that are thought to have split off before the division into monocots and eudicots. They fall under different categories in various classification systems.
Figure 6.2. Key characteristics that separate monocots and dicots. Source: https://viva.pressbooks.pub/introbio2/chapter/8-5-seedplants- angiosperms /
1. Basal Angiosperms
The Magnoliidae category encompasses distinctive plant groups. Magnolias, tall trees with fragrant, multipart flowers, are considered ancient. Laurel trees yield aromatic leaves and inconspicuous flowers. Laurales, typically small trees and shrubs, thrive in warmer climates. Notable examples include bay laurel, cinnamon, spicebush, and avocado trees. Nymphaeales comprise water lilies, lotus, and similar aquatic plants. Flourishing in freshwater habitats, these species have floating or submerged leaves. Water lilies, prized for their beauty, have adorned ponds for millennia. Piperales encompass herbs, shrubs, and small trees in tropical regions. They feature small, petal-less flowers arranged in long spikes. Many species offer valued fragrances or spices, like the black peppercorns from Piper nigrum, enhancing various dishes.
2. Monocots Monocot plants are primarily characterized by having a single cotyledon in their seedlings. They also share anatomical traits like leaves with veins running parallel to their length and flowers exhibiting three- or six-fold symmetrical arrangements. Monocots typically lack true woody tissue, though exceptions like palm trees form trunks from vascular and parenchyma tissues. The pollen of the earliest angiosperms had a single furrow, a feature still present in contemporary monocots. The stem's vascular tissue lacks a specific arrangement, and the root system is often adventitious with no prominent taproot. Well-known monocot plants include true lilies, orchids, grasses, and palms. Many vital crops belong to this group, including rice, corn, sugar cane, bananas, and pineapples.
3. Eudicots True dicots, or eudicots, are distinguished by having two cotyledons in the growing shoot. In leaves, veins create a network, and flower parts have four, five, or more whorls. In dicots, the vascular tissue is dispersed throughout the stem, forming a ring in the stem. Eudicots can generate woody tissues or be herbaceous (like grasses). The majority of eudicots generate trisulcate or triporate pollen, which has three furrows or pores. One major root that originated from the embryonic radicle often serves as the anchor for the root system. Two-thirds of all blooming plants are eudicots. Because many species have traits common to both groups, it is not always easy to determine if a plant is a monocot or a eudicot.
What are Eudicots? Eudicots or true dicots, are characterized by the presence of two cotyledons.
Self-Assessment Exercises 2
1. What is the diversity of angiosperms?
2. What are the divisions in angiosperms?
6.3.4 The Role of Seed Plants
Mosses and liverworts are often the first organisms to colonize new areas, whether in primary successions or after catastrophic events. Their spores are dispersed by wind, birds, or insects. Once established, they offer food and shelter for other plants. In harsh environments like the tundra, bryophytes thrive due to their rootless nature and ability to quickly rehydrate. They form the base of the tundra's food chain, providing sustenance for various species. Bryophytes also enhance soil conditions for other plants, thanks to their relationships with nitrogenfixing cyanobacteria that enrich the soil.
Urban pollution impacts mosses due to their lack of protective features, like a cuticle. Their decline serves as an indicator of environmental pollution. Ferns contribute by aiding rock weathering, soil creation, and erosion prevention. Water ferns restore vital nitrogen to aquatic habitats through symbiosis. Seedless plants, like peat moss, have had historical uses as tools, fuel, and medicine. Plants overall maintain ecosystems by stabilizing soils, carbon cycling, and climate moderation. They provide habitat, food, and resources for various life forms, including humans. Plants are essential to the balance of terrestrial ecosystems and human life.
Some of the roles of seeds and plants include:
1. Providing shelter to many lives forms
2. Providing food for herbivores, thereby indirectly feeding carnivores
3. Providing plant secondary metabolites for medicinal purposes and industrial production
4. Providing wood, paper, textiles, and dyes for human use
5. Providing ornamental species for decorations and inspiration in the arts
Self-Assessment Exercises 3
1. Why are angiosperms called flowering plants?
2. What are the major groups in the classification of angiosperms?
6.4 Summary
In this Unit, you have learnt that gymnosperms and angiosperms are seed producing vascular plants. The efficient seed dispersal of seed plants accounts for their continued existence and widespread occurrence. The distinguish features of Angiosperm in the possession of flower and fruits. Gymnosperms are mostly woody plants. Conifers are of immense economic value primary for timber and paper production. Seed plants do not need immediate aquatic habitat. Seed Plants
End of module questions
1). Describe the characteristics features of the plant kingdom
2). Describe the criteria and the classification of the plant kingdom
3). Describe the adaptations that allowed plants to colonize the land
4). Describe the timeline of plant evolution and the impact of land plants on other living things
5). Discuss the ecological adaptations of the algae
6). Chart the development of land adaptations in the bryophytes
7). Identify the new traits that first appear in seedless tracheophytes
8). Explain the role of seedless plants in the ecosystem
MODULE 3 SURVEY OF THE ANIMAL KINGDOM
Unit 1 Diversity of Animal Life
Unit structure
1.1 Introduction
1.2 Intended Learning Outcomes (ILOs)
1.3 Main Contents
1.3.1 Complex Tissue Structure
1.3.2 Animal Reproduction and Development
1.3.3 Diversity of Animal Life
1.3.3.1 Animal Characterization Based on Body Symmetry
1.3.3.2 Animal Characterization Based on Features of
Embryological Development
1.4 Summary
1.5 Self-Assessment Exercises
1.1 Introduction
Animal evolution began more than 600 million years ago in the ocean, originating from small organisms unlike present-day creatures. The animal kingdom has evolved into a highly diverse group, with over one million identified species and ongoing discoveries. Estimates place the number of extant species between 3 and 30 million. Defining what constitutes an animal can be challenging, as organisms like corals and sponges are less straightforward to classify compared to familiar animals like dogs or birds. Animals encompass a wide range of complexity, and scientists classify them based on shared traits, anatomy, morphology, evolutionary history, embryological development, and genetics. This classification system evolves with new species knowledge. Accurately categorizing diverse species enhances our comprehension of life's variety and aids in conserving Earth's biodiversity.
1.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to do the following:
1. List the features that distinguish the kingdom Animalia from other kingdom
2. Explain the processes of animal reproduction and embryonic development
3. Explain the differences in animal body plans that support basic animal classification
4. Compare and contrast the embryonic development of protostomes and deuterostomes
1.3.1 Complex Tissue Structure
Specialized animal tissues have evolved to fulfill specific functions related to finding and processing food, responding to the environment, movement, and communication. Sensory structures aid navigation and detecting food, while complex digestive systems are supported by accessory organs. Muscle tissues, along with nerve tissues, allow animals to rapidly sense and respond to changes, enabling survival and competition. Unlike plant and fungi cells, animal cells lack cell walls but may be surrounded by an extracellular matrix. Vertebrates have unique supportive tissues like bone. Epithelial tissues cover and protect surfaces, including skin and linings of organs. Animal tissues' differentiation and specialization drive their incredible diversity. The animal kingdom is divided into five monophyletic clades: Porifera (sponges), Placozoa, Cnidaria (jellyfish and relatives), Ctenophora (comb jellies), and Bilateria (all other animals). Placozoa and Porifera lack specialized tissues from germ layers but have cells resembling tissues. Ctenophores are distantly related to Cnidarians and Bilateria in the Eumetazoa ("true animals"). Most animals belong to Eumetazoa, showcasing the diversity of multicellular animals and their distinct tissues derived from germ layers. Evolution of which two tissues resulted in nimals’ unique ability to rapidly sense and respond to changes?
Self-Assessment Exercises 1
1. How does the tissue of animals differ from those of the other major multicellular eukaryotes, plants and fungi?
1.3.2 Animal Reproduction and Development
The majority of animals are diploid creatures, which means that their somatic (body) cells are diploid and their gametes are created by meiosis. There are certain outliers, such as the male bees, wasps, and ants, which originate from unfertilized eggs and are haploid. The majority of animals reproduce sexually. However, several species, including cnidarians, flatworms, and roundworms, are capable of asexual reproduction, in which the progeny derive from a portion of the parent species.
1. Processes of Animal Reproduction and Embryonic Development Animal reproduction involves sexual and asexual methods. Sexual reproduction involves the fusion of male and female gametes through fertilization, forming a diploid zygote. Some aquatic animals reproduce asexually through budding or fragmentation. Parthenogenesis, occurring in insects and some vertebrates, results in offspring from unfertilized eggs. In parthenogenesis, offspring are genetically identical to the parent. Development begins with cleavage, a series of mitotic divisions, resulting in blastomeres. This leads to the formation of a morula, followed by a blastula in some animals. Gastrulation follows, forming the primitive gut and germ layers (endoderm, ectoderm, and in triploblastic animals, mesoderm), which specialize into various tissues, organs, and systems through organogenesis. Diploblastic animals have two germ layers, while triploblastic animals have three. These processes dictate the development of animal morphology and physiology.
Figure 1.1 Embryonic development. Source: https://openstax.org/books/biology/pages/1-introduction.
2
Self-Assessment Exercises
1. What does it meant to say animals are diploid organisms?
2. Briefly describe the processes of animal reproduction and embryonic Development
1.3.3 Diversity of Animal Life
All animals in the animal kingdom are categorized by scientists, while there are several deviations to the majority of the "rules" regulating animal classification. Animals have historically been categorized based on two traits: body layout and developmental pathway. The body plan's symmetry, or how the body parts are placed along the main body axis, is its most notable characteristic. Animals that are symmetrical can be split into about equal halves along at least one axis. The number of germ tissue layers formed during development, the origin of the mouth and anus, the presence or absence of an internal body cavity, and other
aspects of embryological development, such as the type of larva or whether or not growth periods are broken up by molting, are examples of developmental characteristics.
1.3.3.1Animal Characterization Based on Body Symmetry
True animals can be broadly categorized into three groups based on the symmetry of their body plans: radially symmetrical, bilaterally symmetrical, and asymmetrical, at the most fundamental level of classification. Two contemporary clades, the Parazoa and Placozoa, exhibit asymmetry. (However, it should be noted that the Parazoa's ancestors may have shown bilateral symmetry.) Radial or biradial symmetry is present in one clade, the Cnidaria; ctenophores have rotational symmetry. The biggest group, the Bilateria, exhibits bilateral symmetry; however, the Echinodermata are bilateral as larvae and undergo secondary metamorphosis to become radial adults. Each sort of symmetry is ideally suited to fit the special requirements of an animal's lifestyle.
Radial symmetry involves body parts arranged around a central axis, like a bicycle wheel or pie. Animals with radial symmetry have top and bottom surfaces but lack left and right sides or front and back. If divided along the oral/aboral axis, the halves of a radially symmetrical animal are mirror images. This symmetry is common in Cnidaria, such as jellyfish and adult sea anemones, aiding creatures that experience their environment from all directions. Bilateral symmetry, found in animals like butterflies, has one division plane creating equivalent halves. Ctenophora, resembling jellyfish, have rotational symmetry, with one body half rotated 180 degrees.
Figure 1.2 Symmetry in animals. The (a) sponge is asymmetrical. The (b) jellyfish and (c) anemone are radially symmetrical, the (d) butterfly is bilaterally symmetrical. Rotational symmetry (e) is seen in the ctenophore Beroe, shown swimming open-mouthed. Source: https://openstax.org/books/biology/pages/1-introduction.
Bilateral symmetry involves dividing an animal along a midsagittal plane, resulting in two mirror image halves (right and left). This symmetry is present in creatures like butterflies, crabs, and humans. Animals with bilateral symmetry have distinct anterior and posterior, dorsal and ventral, and right and left sides. Most animals in the Eumetazoa group have bilateral symmetry, promoting directional motion and cephalization (organized nervous system in the anterior end). This symmetry allowed for active mobility, resource-seeking, and predator-prey interactions. Some animals like echinoderms display modified radial symmetry as adults but have a bilateral symmetry during their larval stages.
1.3.3.2 Animal Characterization Based on Features of Embryological Development
Most animal species undergo embryonic development with germ layers that give rise to specific tissues and organs. Animals can be classified into two categories based on the number of germ layers they have: diploblasts and triploblasts. Diploblasts, which include animals with radial, biradial, or rotational symmetry, have two germ layers - an inner endoderm or mesendoderm, and an outer ectoderm. These animals lack a coherent third layer of tissue. Triploblasts, found in more complex animals with bilateral symmetry, have three germ layers - endoderm, ectoderm, and mesoderm. These layers differentiate into various tissues and organs during development.
Figure 1.3 Diploblastic and triploblastic embryos. Source: https://openstax.org/books/biology/pages/1-introduction.
Diploblastic and triploblastic embryos.
During embryonic development, diploblasts develop two germ layers: an ectoderm and an endoderm or mesendoderm. Triploblasts, on the other hand, have a third layer known as the mesoderm, which arises from mesendoderm and is situated between the endoderm and ectoderm. These germ layers have specific roles in forming various body tissues and organs. The endoderm gives rise to the lining of the digestive and respiratory tracts, as well as other structures. The ectoderm contributes to the outer covering of the body, the central nervous system, and more. The mesoderm, unique to triploblasts, gives rise to muscle tissues, connective tissues, and visceral organs like the skeleton, blood cells, and internal organs. Diploblastic animals may have cells that serve multiple functions, such as covering and contraction.
Presence or Absence of a Coelom
In triploblasts, there is a subdivision between animals that develop an internal body cavity derived from the mesoderm, known as a coelom, and those that do not. The coelom is a fluid-filled cavity lined with epithelial cells that houses various organs like the digestive, urinary, and reproductive systems, as well as the circulatory system. This coelomic cavity has functional advantages such as shock absorption, organ flexibility, and optimal placement. Animals without a coelom are called acoelomates, while those with a true coelom developed entirely from the mesoderm are eucoelomates. Pseudocoelomates have a partially mesoderm- and partially endoderm-lined coelom. The presence and characteristics of the coelom play a role in the classification and functional attributes of different animal groups.
Figure 1.4 Body cavities. Triploblasts may be (a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, in which both the gut and the body wall are lined with mesoderm. Pseudocoelomates also have a body cavity, but only the body wall is lined with mesoderm. Source: https://openstax.org/books/biology/pages/1-introduction.
Embryonic Development of the Mouth
Bilaterally symmetrical eucoelomates can be categorized into Protostomes and Deuterostomes based on differences in the origin of the mouth during embryonic development. In Protostomes, the mouth forms at the blastopore, while in Deuterostomes, the mouth forms at the opposite end of the gut and the anus develops at the blastopore. These classifications also influence the method of coelom formation. In protostomes, the coelom forms through schizocoely, where specific blastomeres migrate and create mesodermal tissue clumps that eventually develop into cavities forming the coelom. In deuterostomes, coelom formation occurs through enterocoely, as mesoderm pouches pinch off from the endoderm tissue and expand to create the coelom. Another distinction lies in cleavage patterns. Protostomes undergo spiral cleavage, with cells at one pole misaligned with those at the opposite pole due to oblique cleavage angles. Deuterostomes experience radial cleavage, where cleavage axes are parallel or perpendicular to the polar axis, resulting in aligned cells between the poles. These developmental differences reflect broader distinctions in the organization and embryonic development of protostomes and deuterostomes.
Figure 1.5 Protostomes and deuterostomes. Source: https://openstax.org/books/biology/pages/1-introduction.
Eucoelomates can be categorized based on their early embryonic development. In protostomes, the mouth forms near the blastopore, and the body cavity originates through the process of schizocoely, where mesodermal mass splits. Deuterostomes, on the other hand, have the mouth forming opposite the blastopore, and their coelom develops through enterocoely, with mesoderm pouches pinching off. Another distinction lies in the cleavage process. Protostomes undergo determinate cleavage alongside spiral cleavage, where each embryonic cell's fate is predetermined. If a blastomere is removed, structures might be missing, and the embryo might fail to develop. Deuterostomes experience indeterminate cleavage, where cells are not yet committed to specific cell types. Removing blastomeres doesn't lead to loss of structures, and even twins can develop from separated blastomeres. Deuterostomes can compensate for damaged cells through adjacent undetermined cells. This characteristic is reflected in embryonic stem cells, which can develop into various cell types until their fate is determined later in development. What do we refer to Triploblasts that do not develop a coelom?
Self-Assessment Exercises 3
1. What is the difference in organization of protostome and deuterostome embryos from cleavage point of view?
2. What are the three types of Triploblasts coelomates?
Summary
Animals encompass a vast array of organisms, varying from simple sea sponges to humans, yet they share fundamental traits. They are eukaryotic, multicellular, and heterotrophic, obtaining nutrients by consuming other organisms. Most animals develop mobility and adhere to a specific body plan. Notably, animals possess specialized tissues like nerve, muscle, and connective tissues, optimized for specific functions. Sexual reproduction is common among animals, resulting in similar embryonic stages across the kingdom. A set of regulatory genes known as Hox genes plays a pivotal role in shaping major animal body plans, displaying strong homology throughout the animal kingdom.
UNIT 2 THE SIMPLEST ANIMALS
Unit Structure
2.1 Introduction
2.2 Intended Learning Outcomes (ILOs)
2.3.1 Sponges
2.3.1.1 Characteristics of Sponges
2.3.1.2 Physiological Processes in Sponges
2.3.2 Diversity Sponges
2.3.2.1 Habitat and Adaptation of Sponges
2.3.2.2 Comparison to Other Phyla
2.3.3 Cnidarias
2.3.3.1 Characteristics of Phylum Cnidaria
2.3.3.2 Physiological Processes of Cnidarians
2.3.3.3 Diversity of Cnidaria
2.3.3.4 Habitat and Adaptation
2.4 Summary
2.1 Introduction
The majority of animal species on earth are classified as invertebrates. They come in a very wide variety, including scorpions, octopuses, and centipedes. Some of their smallest members are the most fascinating despite their extreme diversity. The most basic animals are the sponges and cnidarians. Sponges appear to be the first multicellular organisms in the animal group. They lack real tissues, in which specialized cells are grouped according to their functional roles, even if they have cells that are specialized for specific functions. Sponges resemble the colonial, flagellated protists that are thought to have existed before mammals. Despite having only two tissue layers, the cnidarians, or jellyfish and their relatives, are the most basic animal group to exhibit real tissues.
2.2 Intended Learning Outcomes
1. Describe the organizational features of the simplest multicellular organisms
2. Explain the various body forms and bodily functions of sponges
3. Identify the common characteristics of phylum Cnidaria
4. Identify common structural and organizational characteristics of the phylum Cnidaria
5. Identify the common characteristics of superphylum Lophotrochozoa 6. Identify the common characteristics of superphylum Ecdysozoa
7. Identify the common characteristics of superphylum Deuterostomia
2.3.1 Sponges
Invertebrates are animals without bony structures like cranium and vertebrae. The simplest invertebrates, Parazoans, encompass only the phylum Porifera, which includes sponges. Parazoans lack tissue-level organization but possess specialized cells for specific functions. While not having true tissues like more advanced animals, sponges still exhibit specialized cell groups that function like tissues. Sponges have body structures designed to facilitate water movement, crucial for functions such as feeding, excretion, and gas exchange. Despite not forming true tissues during embryogenesis, sponges have adapted to their aquatic environments with structures like canals and chambers that allow water to flow and enable essential exchanges within their bodies
2.3.1.1Characteristics of Sponges
Sponges, a diverse group of invertebrates, encompass around 5000 species globally, primarily in marine environments with some freshwater species. They lack true tissues but have specialized cells for various functions. Water movement in sponges is facilitated by flagellabearing choanocytes that create currents through pores and canals. They can be radially symmetrical or asymmetrical, supported by a protein collagen and spicule skeleton embedded in a gelatinous matrix. Sponges capture food via water currents created by choanocytes and digest it within individual cells. Reproduction occurs sexually and asexually; gemmules, internal buds, aid survival in unfavorable conditions. In sexual reproduction, gametes are captured by choanocytes and transferred to eggs. Sponges exhibit different body plans: asconoid (simple tube), syconoid (larger tube with canals), and leuconoid (complex mass with numerous chambers). They are present in various aquatic habitats, often containing toxic substances for protection and competition. Sponges have potential pharmaceutical uses due to beneficial compounds they produce. They form symbiotic relationships with plants, bacteria, and algae, and some bore into corals and mollusks. Overall, sponges play important ecological roles in marine ecosystems.
2.3.1.1 Physiological Processes in Sponges
Sponges, despite being simple organisms, regulate their different physiological processes through a variety of mechanisms. These processes regulate their metabolism, reproduction, and locomotion.
Digestion
Sponges lack complex digestive, respiratory, circulatory, and nervous systems. They feed by trapping food particles as water flows through their ostia and out the osculum. Choanocytes capture bacteria for ingestion, while larger particles are taken in by pinacocytes on the surface. Amoebocytes transport food to cells that haven't ingested any.
Digestion is intracellular, limiting the size of ingested food particles. Oxygen and carbon dioxide exchange, circulation, and waste excretion occur through diffusion between sponge cells and surrounding water. Some sponges host photosynthetic endosymbionts like algae or cyanobacteria. Around 150 species of carnivorous sponges feed on tiny crustaceans using sticky threads or spicules. While sponges lack a dedicated nervous system, intercellular communication regulates actions like body contraction and choanocyte activity.
Reproduction
Sponges reproduce through both sexual and asexual methods. Asexual reproduction involves fragmentation (breaking off a piece of the sponge that develops into a new individual) or budding (an outgrowth growing from the parent and detaching or forming a colony). Freshwater sponges exhibit an atypical asexual method using gemmules, resistant structures containing archeocytes surrounded by cells and spicules. Gemmules survive harsh environments and later colonize habitats. Sexual reproduction involves gamete formation. Oocytes develop from
amoebocytes and are retained, while spermatozoa result from choanocyte differentiation and are expelled. Sponges are monoecious (hermaphroditic), producing both eggs and sperm. Gamete production varies with environmental conditions, and some sponges become sequentially hermaphroditic. Cross-fertilization is encouraged by the temporal separation of gametes. Spermatozoa from one sponge fertilize oocytes in others. Early larval development occurs within sponges, with free-swimming larvae released via the osculum.
Locomotion
As adults, sponges are often sessile and reside linked to a permanent substrate. They do not exhibit long-distance locomotion like other freeswimming sea invertebrates do. Sponge cells may, however, saunter around surfaces because to their organizational flexibility, or ability to rearrange their cells. Researchers have demonstrated that sponge cells spread out on a physical substrate have a leading edge for directed movement under experimental circumstances. It has been hypothesized that this restricted creeping motion aids sponges in adjusting to the microenvironments close to the attachment point. However, it should be highlighted that while this pattern of movement has been observed in lab settings, it has not yet been confirmed in environments where sponges naturally live. Are porifera only sponges?
Self-Assessment Exercises 1
1. What is the mode of nutrition in sponges?
2. Why are Poriferans confused to be plants instead of animals?
2.3.2 Diversity Sponges
Sponges are diverse invertebrates classified into four classes: calcareous sponges (Calcarea), glass sponges (Hexactinellida), demosponges (Demospongiae), and encrusting sponges (Homoscleromorpha). Different classes are distinguished by the presence and composition of spicules and spongin. Demosponges are the most common, comprising 90% of all sponge species, with variable spicules and spongin. Calcareous sponges have calcium carbonate spicules or exoskeletons, and are found in shallow marine waters. Homoscleromorpha sponges are simple and encrusting, with small, uniform spicules. Hexactinellid sponges have strong, lattice-like silica skeletons, often with a cupshaped form.
Figure 2.1 Sponge Spicule: Sponges are classified based on the presence and types of spicules they contain. Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
Figure 2.2 Types of sponges: (a) Clathrina clathrus belongs to class Calcarea, (b) Staurocalyptus spp. (common name: yellow Picasso sponge) belongs to class Hexactinellida, and (c) Acarnus erithacus belongs to class Demospongia.Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
Poriferans, or sponges, are multicellular organisms that lack true tissues and organs, and they do not exhibit body symmetry. They possess specialized cells that perform specific functions. Their bodies are adapted for efficient water flow, which is crucial for feeding, gas exchange, and excretion. They have pores called ostia through which water enters and canals, chambers, and cavities that enable water to reach most cells.
Sponges typically reproduce sexually, releasing sperm into the water to fertilize eggs. Fertilized eggs develop into larvae that search for suitable settling places. Asexual reproduction occurs through fragment regeneration, budding, and the formation of gemmules, dormant structures that allow survival in harsh conditions. Many of the thousands of sponge species are filter-feeders, consuming bacteria and particles from the water. Some species are carnivorous, preying on small crustaceans, while others host photosynthetic microorganisms as endosymbionts, benefiting from the produced food and oxygen.
2.3.2.1Habitat and Adaptation of Sponges
Sponges have a global distribution, from polar to tropical latitudes, and can thrive in diverse habitats ranging from deep ocean depths to shallow rock pools. They are found in marine thermal vents, freezing Arctic waters, and freshwater environments. While most are in marine habitats, some occupy brackish water and freshwater. Sponges inhabit a wide array of ecosystems including reefs, seagrass beds, and ocean trenches, avoiding areas with strong currents or crashing waves. Their distribution spans the entire globe, with species inhabiting cold Arctic waters to warm tropical oceans, as well as various freshwater regions. Sponges have several adaptations that allow them to survive in different environments.
These adaptations include:
1. Body shapes that are adapted for maximal efficiency of water
flow through the central cavity.
2. Internal skeletons of spongin and/or spicules (skeletal-like
fragments) of calcium carbonate or silicon dioxide.
3. Lack of organs and specialized tissue.
4. Flagellated cells that move water into the central cavity through
the perforations.
5. Individual cells that digest food, excrete waste, and absorb
oxygen.
6. Ability to reproduce asexually or sexually.
7. Larval forms that are free-swimming but all adults are sessile.
8. Skeleton types that allow them to live in either hard or soft
sediments.
9. Pores that allow them to filter the water around them for food.
Sponges are strong animals with dense skeletons that are well adapted to their environments. They may live almost everywhere and adapt to the regions and surfaces they grow in. Their skeletal type allows them to live on hard, rocky surfaces or soft sediments such as sand and mud. Certain sponge species are adapted to freshwater environments. Their pores allow them to filter the water around them for food. Where are Porifera sponges found?
1. Comparison to Other Phyla
Sponges themselves are really distinctive creatures and share few similarities with organisms in other phyla. They are the only invertebrates to be asymmetrical and along with cnidarians they are only classified on the tissue level of organization. In fact, although cnidarians are dissimilar in appearance, they share many characteristics with porifera. Both are sessile at one point in their life cycles, which also is diplontic. Porifera are one of the oldest animal ancestors, which alsoleads us to believe that they evolved much earlier than other invertebrates. These animals truly are remarkable and have evidently managed to distinguish themselves immensely form other invertebrates regarding characteristics, adaptations, as well as behaviour.
What are the four types of sponges?
Self-Assessment Exercises 2
1. Describe the different cell types and their functions in sponges.
2. Describe the feeding mechanism of sponges and identify how it is different from other animals.
2. Cnidarias
Porifera are at a simpler level of organization than cnidarians. They have a noncellular mesoglea sandwiched between two layers of tissue on the outside and inside. Cnidarians do extracellular digestion and have a well-developed digestive system. The cnidocyte is a unique cell that can both warn off predators and give toxins to prey. A cnidarian's life cycle includes physically diverse forms, and they have separate sexes. At different times throughout their lives, these animals also exhibit the two distinct morphological forms known as polypoid and medusoid.
2.3.3.1Characteristics of Phylum Cnidaria
Phylum Cnidaria consists of diploblastic animals that exhibit radial or biradial symmetry. Nearly all cnidarians are marine species. These organisms possess specialized cells called cnidocytes, which contain stinging organelles called nematocysts. These cells are primarily located around the mouth and tentacles and are used to immobilize prey or deter predators. Nematocysts contain coiled threads with barbs and are activated by touch through hairlike projections called cnidocils. Upon activation, these cells release toxins into the target, aiding in capturing prey or deterring predators.
Figure 2.3. Animals from the phylum Cnidaria have stinging cells called cnidocytes. Cnidocytes contain large organelles called (a) nematocysts that store a coiled thread and barb. When hairlike projections on the cell surface are touched, (b) the thread, barb, and a toxin are fired from the organelle. Source: http://www.fleabites.net/beneficial-nematodes-forfleas-how-they-work/."
The polyp or "stalk" and the medusa or "bell" body plans are two unique cnidarian body types (Figure 2.4). Freshwater Hydra species are examples of the polyp form; jellies (jellyfish) are probably the bestknown medusoid creatures. As adults, polyps are sessile, with only digestive system entrance (the mouth), which is surrounded by tentacles. The mouth and tentacles of a medusa hang from its bell-shaped body, and it is a moving creature. Other cnidarians have both a polyp and a medusa form, and these forms alternate throughout the life cycle.
Figure 2.4 Polymorphic cnidarians. Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
Some cnidarians exhibit polymorphism, displaying two different body plans during their life cycle. An example is the colonial hydroid called Obelia. This hydroid has two types of polyps in its sessile polyp form: gastrozooids for feeding and gonozooids for asexual reproduction of medusae. The medusae, which are free-swimming and either male or female, are produced from reproductive buds on the gonozooids. After maturing, these medusae release sperm or eggs. Fertilization leads to the development of a zygote into a blastula, and then into a planula larva. The larva swims freely for a period before attaching, eventually forming a new colonial reproductive polyp.
Figure 2.5. The sessile form of Obelia geniculate has two types of polyps: gastrozooids, which are adapted for capturing prey, and gonozooids, which bud to produce medusae asexually.Click here to follow the life cycle of the Obelia. Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
Cnidarians exhibit a simple body structure with two main tissue layers: the outer epidermis derived from ectoderm and the inner gastrodermis derived from endoderm. These layers enclose a jelly-like mesoglea. While the cellular complexity is present, the development of organs is limited. Their nervous system is basic, with nerve cells scattered throughout the body, forming nerve nets, plexi, or cords. Despite its simplicity, the nervous system coordinates various functions like tentacle movement and prey capture. Cnidarians perform extracellular digestion, secreting enzymes into their gastrovascular cavity for nutrient absorption. They have an incomplete digestive system with a single opening for both ingestion and egestion. Oxygen and carbon dioxide exchange occurs through diffusion between cells and the environment. Cnidarians lack circulatory and excretory systems, and waste elimination happens through diffusion. Nutrient distribution also relies on diffusion through the mesoglea between cells.
2.3.3. 2Physiological Processes of Cnidarians
Cnidarians possess two tissue layers: the outer epidermis and the inner gastrodermis, separated by a jelly-like mesoglea. These layers contain different cell types and intercellular connections. Despite cellular differentiation, cnidarians lack organs and organ systems. Their basic nervous system consists of scattered nerve cells forming a network that transmits signals from sensory to contractile cells. They engage in extracellular digestion followed by intracellular digestion. Enzymes are secreted into the gastrovascular cavity, and absorbed nutrients aid intracellular digestion. The gastrovascular cavity functions as both a mouth and an anus (incomplete digestive system). Oxygen, carbon dioxide, and nitrogenous waste exchange occur through diffusion between cells and the environment.
2.3.3.3 Diversity of Cnidaria
About 10,000 identified species make up the phylum Cnidaria, which is classified into the Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa classes. The scyphozoans (jellyfish) and cubozoans (box jellies) are swimming forms, in contrast to the anthozoans, sea anemones, and corals, which are all sessile species. The hydrozoans include swimming colonial forms like the Portuguese Man O' War and sessile forms.
1. Class Anthozoa
The class Anthozoa encompasses cnidarians with a solitary polyp body plan, excluding the medusa stage. This group includes sea anemones, corals, and sea pens, totaling about 6,100 species. Sea anemones are cylindrical and attached to substrates, with colorful appearance and tentacles containing cnidocytes for prey capture. Their mouth is surrounded by tentacles and a siphonophore-lined pharynx. The gastrovascular cavity is divided by mesenteries, increasing nutrient absorption and gas exchange. Sea anemones feed on small fish and shrimp, using their cnidocytes to immobilize prey. Some establish symbiotic relationships with hermit crabs and clownfish. Anthozoans reproduce asexually via budding or fragmentation, or sexually by producing gametes. Polyps produce both gametes, which can fuse to generate a planula larva, eventually settling into a sessile polyp form.
Figure 2.6 Cnidaria polyp and medusa forms. Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
Class Scyphozoa includes all the jellies and is exclusively a marine class of animals with about 200 known species. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present. Members of this species range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata, can reach a size of 2 m across. Scyphozoans display a characteristic bell-like morphology (Figure 2.7).
Figure 2.7 A jelly illustrating its morphology. Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
Jellyfish belonging to the class Scyphozoa have a mouth opening surrounded by tentacles bearing nematocysts on their underside. These solitary carnivores spend most of their life cycle as free-swimming organisms. The mouth leads to a gastrovascular cavity divided into interconnected sacs called diverticuli, potentially further branched into radial canals. This structure enhances nutrient absorption and diffusion due to increased contact with the gastrovascular cavity. Nerve cells are scattered throughout the body, and a ring of muscles around the dome enables swimming. Scyphozoans are dioecious, with separate sexes and gonads formed from the gastrodermis. Gametes are expelled through the mouth. They have a polymorphic life cycle, exhibiting both medusal and polypoid body plans. The life cycle involves planula larvae produced through external fertilization, which settle as polypoid forms called scyphistoma. These can give rise to additional polyps through budding or transform into the medusoid form.
Figure 2.8. The life cycle of a jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding, and the medusa reproduces sexually. Source:
Class Cubozoa
The class Cubozoa includes box jellyfish, characterized by their boxshaped medusa or square bell. These species can reach sizes of 15-25 cm. They share similar morphological and anatomical features with scyphozoans. A key distinction is in tentacle arrangement. Cubozoans are highly venomous among cnidarians. They possess muscular pads called pedalia at the corners of the bell, with tentacles attached to each pedalium. Tentacles might have nematocysts arranged spirally for effective prey capture. These creatures are categorized into orders based on the number of tentacles per pedalium. The digestive system can extend into the pedalia. Cubozoans have a polypoid form that arises from a planula larva. These polyps have limited mobility, may bud to create more polyps for habitat colonization, and eventually transform into medusoid forms.
Figure 2.9. A tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies. Source: http://www.fleabites.net/beneficialnematodes-for-fleas-how-they-work/." Class Hydrozoa
The class Hydrozoa comprises around 3,200 species, most of which are marine, with some freshwater species. Hydrozoans are polymorphic, often showing both polypoid and medusoid forms in their life cycle. The polyp form is typically cylindrical, with a central gastrovascular cavity lined by gastrodermis. Tentacles surround a mouth opening at the oral end. Many hydrozoans form colonies composed of specialized polyps sharing a gastrovascular cavity, like in the colonial hydroid Obelia. Some colonies are free-floating and include both medusoid and polypoid individuals, as seen in Physalia and Velella. Other species exist as solitary polyps (Hydra) or solitary medusae (Gonionemus). Notably, their gonads for sexual reproduction are derived from epidermal tissue, unlike other cnidarians where they originate from gastrodermal tissue.
Explain the function of nematocysts in cnidarians. The nematocysts are “stinging cells” designed to paralyze prey. The nematocysts contain a neurotoxin that renders prey immobile.
Figure 2.10 Hydrozoans. The Tubularia indivisa (a), siphonophore colonies Physalia (b) physalis, known as the Portuguese man o‘war and Velella bae (c), and the solitary polyp Hydra (d) have different body shapes but all belong to the family Hydrozoa. Source: http://www.fleabites.net/beneficial-nematodes-for-fleas-how-theywork/."
2.3.3.4 Habitat and Adaptation
Cnidarians exhibit a range of adaptations that suit their diverse habitats. They often attach to solid substrates or burrow into sediments, with polyps being common in shallow waters and even deep ocean environments. Medusae are carried by currents and have preferred depths. Cnidarians are radially symmetrical, lack cephalization, and have two cell layers instead of three like higher animals. Their activities are coordinated by a decentralized nerve net and simple receptors. Some free-swimming Cubozoa and Scyphozoa species have balance-sensing statocysts and simple eyes. Cnidarians were the first animals to possess muscles and nerves for behavior, and they feature specialized structures for pumping and circulating water. Other adaptations of cnidarians include:
1. Gastrovascular cavity (incomplete gut) divided with septa
2. Extracellular digestion
3. Epitheliomuscular cells which help in muscular contractions
4. Well-developed statocysts for balance
5. Ocelli (photosynthetic)
Self-Assessment Exercises
1Compare the structural differences between Porifera and Cnidaria.
2. Compare the differences in sexual reproduction between Porifera and Cubozoans. How does the difference in fertilization provide an evolutionary advantage to the Cubozoans?
UNIT 3 FLATWORMS, NEMATODES, AND ARTHROPODS
Unit Structure
3.1 Introduction
3.2 Intended Learning Outcomes (ILOs)
3.3 Main Contents
3.3.1 Flatworms
3.3.1.1 Characteristic Features of Flatworms
3.3.1.2 Physiological Processes of Flatworms
3.3.1.3 Diversity of Flatworms
3.3.1.4 Habitat and Adaptation
3.3.2 Nematodes
3.4 Summary
3.5 Self-Assessment Exercises
3.1 Introduction
The triploblastic animal phyla in this and the following modules have an embryonic mesoderm in the middle of the ectoderm and endoderm. Additionally, a longitudinal section of these phyla will separate them into right and left sides that are mirror reflections of one another because to their bilateral symmetry. The onset of cephalization, the evolution of a concentration of neurological systems and sensory organs in the organism's head, where the creature first interacts with its environment, is connected to bilateralism.
3.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to:
1. Describe the structure and systems of flatworms
2. Describe the characteristics and physiological processes of flatworms
3. Describe the structural organization of nematodes
4. Compare the internal systems and the appendage specialization of arthropods
3.3.1 Flatworms
Flatworms, or phylum Platyhelminthes, encompass a diverse range of acoelomate organisms with varying lifestyles, including both free-living and parasitic forms. Nematodes, or roundworms, are pseudocoelomate organisms and comprise both free-living and parasitic species.
Arthropods, a highly successful group, are coelomate organisms characterized by a rigid exoskeleton and jointed appendages. Nematodes and arthropods are part of the Ecdysozoa clade, sharing the feature of periodic molting of their exoskeleton. This clade includes various phyla with hard cuticles that need to be shed for growth. Flatworms, or Platyhelminthes, possess three embryonic germ layers that give rise to different tissue types, including epidermal, mesodermal, and digestive lining tissues. Their bodies lack a true coelom and are covered by a layer of circular muscle, longitudinal muscle, and an epidermal layer. Flatworms have various lifestyles, with many being parasitic, and they are of medical importance to humans.
3.3.1.1Characteristic Features of Flatworms
They are bilaterally symmetrical Their body are dorsiventrally flattened; known as flatworms Triploblastic animals – made up of three body layers They lack body cavity hence called Acoelomate They have complete reproductive organs Digestive system is absent in some; and when present has only the mouth but no anus Nervous system are ladder-like, with simple sense organs They have no respiratory, circulatory or skeletal system They have a proto-nephridial type of excretory system.
The phylum platyhelminthes is classified into three main classes. These are.
1`Turbellaria: mostly free-living and aquatic, with soft bodies and leaf like in form. They have body covered with cilias, some are terrestrial and confined to humid areas and with only one opening to the gut. Examples include planaria, etc.
2 Trematoda: They are parasitic; lacking cilia; cuticle covering leaf like body with one or more suckers. Examples include faciola hepatica (also known as liver fluke) schistosoma, also known as blood fluke, etc.
3. Cestoda: They are endoparasites (internal parasites), having no gut (digestive) system. There are parasites in the digestive tracts of various vertebrates. They are Ribbon – like in form made up of many segments (proglottids) with an anterior scolex carrying suckers and hooks to hest tissues. When mature, each prolothic has a complete set of reproductive organs of both sexes. Examples are the Tapeworms like Taenia solium, Taenia saginata etc.What are the three main classes of members of phylum platyhelinthes?
3.3.1.2 Physiological Processes of Flatworms
Flatworms exhibit diverse feeding strategies depending on their lifestyle. Free-living species can be predators or scavengers, while parasitic forms feed on host tissues. Their digestive systems can be incomplete with a mouth used for both intake and waste expulsion, sometimes accompanied by an anal opening. Some flatworms have a simple saclike gut, while others possess a more branched structure. Digestion occurs outside cells, with enzymes secreted into the digestive tract and absorbed by phagocytosis. Cestodes, a specific group of flatworms, lack a digestive system due to their parasitic nature within their host's digestive cavity, where they directly absorb nutrients across their body wall. Flatworms possess an excretory system consisting of tubules and flame cells distributed throughout the body. These cells beat cilia to expel waste fluids, helping regulate salt levels and eliminate nitrogenous waste. Their nervous system includes paired nerve cords along the body connected to a ganglion at the anterior end, often with sensory cells concentrated in this region (Figure 3.1).
Figure 3.1 This planarian is a free-living flatworm that has an incomplete digestive system, an excretory system with a network of tubules throughout the body, and a nervous system made up of nerve cords running the length of the body with a concentration of nerves and photosensory and chemosensory cells at the anterior end. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Since there is no circulatory or respiratory system, gas and nutrient exchange is dependent on diffusion and intercellular junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. Most flatworm species are monoecious (hermaphroditic, possessing both sets of sex organs), and fertilization is typically internal. Asexual reproduction is common in some groups in which an entire organism can be regenerated from just a part of itself.
3.3.1.3 Diversity of Flatworms
Flatworms are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda (Figure 15.16). The turbellarians include mainly free-living marine species, although some species live in freshwater or moist terrestrial environments. The simple planarians found in freshwater ponds and aquaria are examples. The epidermal layer of the underside of turbellarians is ciliated, and this helps them move. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.
Figure 3.2 Phylum Platyhelminthes is divided into four classes: (a) Bedford’s Flatworm (Pseudobiceros bedfordi) and the (b) planarian belong to class Turbellaria; (c) the Trematoda class includes about 20,000 species, most of which are parasitic; (d) class Cestoda includes tapeworms such as this Taenia saginata; and the parasitic class Monogenea (not shown). Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Monogeneans are external parasites primarily found on fish, with a simple life cycle involving attachment to a single fish host. They may digest host tissues or feed on surface mucus and skin particles. Most are hermaphroditic, mating between individuals rather than self-fertilizing. Trematodes, or flukes, are internal parasites with complex life cycles involving mollusk primary hosts and other secondary hosts. Some cause serious human diseases like schistosomiasis, infecting millions and causing organ damage. Cestodes, or tapeworms, are internal parasites of vertebrates. They live in the host's intestinal tract, anchored by a scolex and comprising proglottids for reproduction. Tapeworms lack a digestive system, absorbing nutrients from the host's food. Reproduction occurs in mature proglottids, which detach and are released in host feces. Tapeworms have a cycle involving intermediate hosts and often infect humans through undercooked meat consumption.
3.3.1.4 Habitat and Adaptation
The liver fluke Fasciola hepatica primarily infects sheep and other vertebrates, residing in their liver and bile passages. It can also infect other animals like goats, dogs, cows, deer, rabbits, elephants, and even humans. It has a complex life cycle involving an intermediate mollusk host and a primary vertebrate host. The adult fluke causes liver damage in its host and can lead to liver disease. The fluke's body is protected by a tough cuticle. It possesses a suctorial pharynx for feeding, and its alimentary canal distributes nutrients using interlinary caeca. The excretory system is prolonephridial, composed of flame cells that remove waste via cilia-driven excretory capillaries. Respiration occurs through the general body surface. The liver fluke is hermaphroditic, containing both male and female reproductive organs in the same individual.
Self-Assessment Exercises 1
1. What are the four traditional classes of flatworms?
2. What are the structural adaptations of the flatworms?
3.3.2 Nematodes
The phylum Nematoda, or roundworms, includes more than 28,000 species with an estimated 16,000 parasitic species. The name Nematoda is derived from the Greek word “nemos,” which means “thread.” Nematodes are present in all habitats and are extremely common, although they are usually not visible (Figure 3.3).
Figure 3.3 (a) An scanning electron micrograph of the nematode Heterodera glycines and (b) a schematic representation of the anatomy of a nematode are shown. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Most nematodes share a similar appearance: elongated tubes with tapered ends (Figure 3.3). Nematodes belong to the pseudocoelomate group and possess a complete digestive system with distinct mouth and anus openings. These organisms are covered by a flexible but sturdy external skeleton called a cuticle, which provides protection and structural support. This cuticle contains chitin, a carbohydrate-protein compound, and it lines both the pharynx and rectum. Although the cuticle safeguards the nematode, it hinders growth, necessitating regular shedding and replacement as the organism grows larger. At the front end, a nematode's mouth is equipped with three or six lips, sometimes including cuticular extensions that resemble teeth. Additionally, some species feature a pointed stylet that can extend from the mouth to pierce prey or plant and animal cells. The mouth leads to a muscular pharynx and intestine, eventually leading to the rectum and an anal opening at the rear end
3.3.2.1Characteristics of Nematods
The Phylum Nematoda, commonly known as roundworms, encompasses
a diverse group of organisms with approximately 12,000 described species, though the actual number could be much higher. Nematodes are abundant in various habitats, including terrestrial, freshwater, and saltwater environments, as well as within other organisms as parasites.
They display a range of feeding habits and ecological roles, contributing significantly to decomposition and nutrient cycling. Nematodes have a slender, unsegmented body that tapers at both ends and a round cross-section. Their external cuticle provides protection and support, which is shed periodically during growth. The hypodermis underneath the cuticle is a syncytium with muscle cells beneath it. Nematodes lack a true coelom, and their internal cavity isn't lined by cells derived from the mesoderm.
The nervous system of nematodes consists of a nerve ring around the pharynx and longitudinal nerve cords along the body. Sensory receptors are present at both ends, including tactile and chemosensory cells. They lack complex eyes but may possess light-sensitive organs. Nematodes have a complete digestive system with a muscular pharynx, and nutrient absorption occurs in the intestine. Waste elimination and gas exchange happen through the body surface, which is feasible due to their small size.
Most nematodes are dioecious, with separate male and female reproductive organs, while some are hermaphroditic. Mating involves specialized structures in males, and sperm lacks flagella, moving through amoeboid motion. Nematodes can be either live-bearing or egglaying, with eggs often escaping through a gonopore in females. They lack a distinct larval stage, with eggs directly developing into juveniles that closely resemble adults. Nematodes exhibit a unique feature called "eutely," where individuals of a species have the same number of cells, achieved during development, leading to growth through cell size increase rather than cell number increase.
3.3.2.2 Physiological Processes of Nematodes
In nematoouter layer, there are longitudinal muscles enabling side-to-side body movement.
Nematodes exhibit various sexual reproductive strategies depending on the species. They can be monoecious (both sexes in one individual), dioecious (separate sexes), or reproduce asexually through parthenogenesis. The species Caenorhabditis elegans is unique with both self-fertilizing hermaphrodites and a male sex that can mate with the hermaphrodite.
3.3.2.3 Diversity of Nemadotes
The phylum Aschelminthes is divided into five classes: Rotifera, Gastrotricha, Echinodera, Priapulide, and Nematode. Rotifers are marine parasites with a ciliary organ called corona on their anterior end. Gastrotricha are free-living, unsegmented organisms found in water with algae and debris, often hermaphroditic. Echinodera are small marine worm-like animals, generally dioecious. Priapulides are marine animals with a proboscis and trunk, also dioecious. Nematodes are unsegmented roundworms with a cylindrical body, having separate sexes, and the males are smaller than females.
These organisms are bilaterally symmetrical with a radial tendency along the longitudinal axis. They have circular cross-sections, lack segmentation and appendages, and possess a complex cuticle. Tissues and organs are present, and the body has more than two cell layers. Circular muscles are absent from the body wall, and the body cavity is a pseudocoel under high pressure. The digestive system extends from the anterior mouth to the anus. Muscles in the body wall have distinctive features. There's no circulatory system, flame cells, or nephridia. Cilia and flagella are absent. Development is direct, involving an increase in cell size rather than cell number.
1. Habitat and Adaptation
Nematodes can inhabit a wide range of environments, including cultivated fields, forests, grasslands, deserts, tundras, and ocean beaches. They can thrive at varying elevations, from high to low. A single square yard of soil might contain more than a million nematodes, making them the dominant organisms in many terrestrial ecosystems. These creatures constitute around 90% of all life forms found on the ocean floor. They provide benefits to diverse ecosystems for various reasons. Nematodes possess a remarkable ability to withstand extreme dryness, known as desiccation, due to their abundance of late embryogenesis proteins. In soil environments, many nematodes prefer areas rich in nutrients and bacteria, where they feed on decomposing organic matter. Their wide range of sizes and uncomplicated body structures enables them to function effectively in virtually every ecosystem. Nematodes are also widely distributed on the ocean floor, participating in the decomposition of various substances in aquatic environments. Their small size, sometimes microscopic, allows them to carry out decomposition processes throughout the earth's lithosphere, which is the outermost crust. Nematodes are pivotal in the breakdown of soil nutrients, contributing significantly to soil fertility.
Figure 3.4 Nematode habittats: Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Nematodes are bilaterally symmetrical, elongate, and usually tapered at both ends. Some species possess a pseudocoel, a fluid-filled body cavity between the digestive tract and the body Nematodes are a highly diverse group of organisms that show a variety of adaptations to extremes in soil and plant environments. Some of the unique adaptations of nematodes include:
1. Epidermis (skin) is not made of cells, but a mass of cellular material and nuclei without separate membranes.
2. Muscle cells only run in longitude direction.
3. Nematodas can only bend their body's from side to side, so when free-swimming, it thrashes its body from side to side, trying to move.
4. You can find 90,000 individual nematodes in a rotting apple.
Self-Assessment Exercises 2
1. How is nitrogenous wastes removed in nematodes?
2. What are the divisions of phylum Aschelminthes?
3.3.3 Arthropods
The word "arthropoda" literally translates as "jointed legs," which perfectly sums up each of the countless species that make up this phylum. With an estimated 85% of known species in the animal kingdom, including many that are still undiscovered or unrecorded, arthropods dominate the animal kingdom. The functional segmentation of the body and the existence of jointed appendages are the defining traits of all the creatures in this class (Figure 3.5). Arthropods, which are Ecdysozoa, also have an exoskeleton that is primarily formed of chitin. In terms of species count, Arthropoda is the animal kingdom's greatest phylum, and insects make up its single largest group. True coelomate creatures, arthropods display prostostomic development.
3.3.3.1Characteristic features of Arthropods
Arthropods, members of the phylum Arthropoda, are a diverse group of animals including insects, crustaceans, spiders, scorpions and centipedes. However, the members of this phylum, despite their incredible diversity and sheer numbers, share a number of important distinguishing characteristics.
Exoskeleton
Arthropods are invertebrates, which means their bodies do not have internal bones for support. To compensate for this, they produce a hard exoskeleton made of chitin, a mixture of lipids, carbohydrates and protein, which covers and protects their bodies like a suit of armor. As arthropods grow, they must shed or molt their exoskeletons. They first produce new, softer exoskeletons underneath the old ones. Once their hardened, old coverings crack and shed, they sport roomier, albeit soft, exoskeletons. Arthropods are incredibly vulnerable during the molting process, and will often hide until their new exoskeletons harden.
Segmented Bodies
Arthropods have bodies that are internally and externally segmented. The number of segments depends on the individual species; millipedes, for example, have more segments than lobsters.
Jointed Appendages
The name arthropod actually comes from the Greek “arthro,” meaning joint, and “pod,” meaning foot. All arthropods have jointed limbs attached to their hard exoskeletons that allow for flexibility and movement. The joints generally bend in only one direction but allow for sufficient predatory and defensive actions.
Bilateral Symmetry
An arthropod's body can be divided vertically into two mirror images. This is called bilateral symmetry. An arthropod shares this symmetry with many other animals such as fish, mice and even humans. Other animals such as the jellyfish and sea star exhibit radial symmetry, while coral and sea sponge are asymmetrical -- exhibiting no pattern at all.
Open Circulatory System
An arthropod has an open circulatory system. This means instead of a closed circulatory system of interconnected veins and capillaries, an arthropod’s blood is pumped through open spaces called sinuses in order to reach tissues. An arthropod does, however, have a heart which pumps
blood into the hemocoel, the cavity where the organs are located, where it surrounds the organs and tissues.
3.3.3.2 Physiological Processes of Arthropods
Arthropods are characterized by their segmented body structure, where certain segments fuse to form functional units like the head, thorax, and abdomen or other combinations. They possess a hemocoel (blood cavity) as their coelom and have an open circulatory system. This system involves a two-chambered heart that pumps blood, which directly bathes internal organs instead of flowing through vessels. The respiratory systems in arthropods vary: insects and myriapods use tracheae tubes branching throughout their bodies and opening through spiracles for direct gas exchange with cells. Aquatic crustaceans have gills, arachnids utilize internal "book lungs" resembling stacked pages, and aquatic chelicerates employ external "book gills" made of leaf-like structures for gas exchange with surrounding water (Figure 3.4).
Figure 3.5 The book lungs of (a) arachnids are made up of alternating air pockets and hemocoel tissue shaped like a stack of books. The book gills of (b) crustaceans are similar to book lungs but are external so that gas exchange can occur with the surrounding water. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
3.3.3.3Arthropod Diversity
The phylum Arthropoda consists of organisms that have successfully adapted to various environments including land, water, and air. This phylum is subdivided into five subphyla: Trilobitomorpha (trilobites), Hexapoda (insects and their relatives), Myriapoda (millipedes, centipedes, and similar creatures), Crustacea (crabs, lobsters, etc.), and Chelicerata (horseshoe crabs, arachnids, etc.). Trilobites, now extinct, existed from the Cambrian to Permian periods, and are possibly most closely related to Chelicerata. There are around 17,000 identified species of trilobites through fossils.
Hexapoda, as indicated by their name, possess six legs or three pairs. Their segments are fused into distinct regions like the head, thorax, and abdomen. The thorax holds both wings and three pairs of legs. Examples of Hexapoda include everyday insects such as ants, butterflies, bees, and cockroaches. What are the classes into which the phylum Arthropoda is divided? What are the three main classes and some of their representative species?
Figure 3.6 In this basic anatomy of a hexapod, note that insects have a developed digestive system (yellow), a respiratory system (blue), a circulatory system (red), and a nervous system (purple). Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Subphylum Myriapoda includes arthropods with legs that may vary in number from 10 to 750. This subphylum includes 13,000 species; the most commonly found examples are millipedes and centipedes. All myriapods are terrestrial animals and prefer a humid environment (Figure 3.5)
Figure 3.7 (a) The centipede Scutigera coleoptrata has up to 15 pairs of legs. (b) This North American millipede (Narceus americanus) bears many legs, although not one thousand, as its name might suggest. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51ef14f21b5eabd@10.8
Aquatic arthropods known as crustaceans, including species like shrimp, crabs, lobsters, and crayfish, dominate their habitat. A handful of crustaceans are adapted to live on land, such as pill bugs and sow bugs. The currently recognized number of crustacean species is around 47,000. While the fundamental body structure of crustaceans resembles that of Hexapoda, involving a head, thorax, and abdomen, in certain species, the head and thorax can merge to create a cephalothorax, which is shielded by a plate called the carapace. Furthermore, the exoskeleton of many crustaceans is strengthened with calcium carbonate, making it more robust than that of other arthropods. Crustaceans operate with an open circulatory system, where blood is pumped into the hemocoel by the dorsal heart. Although most crustaceans have distinct genders, a few, like barnacles, may be hermaphroditic. Some crustacean species exhibit serial hermaphroditism, where their gonads can switch from producing sperm to producing eggs. Many crustaceans experience larval stages during their early development. While the majority of crustaceans are carnivorous, detritivores and filter feeders are also prevalent. What are the main morphological features of arthropods?
Figure 3.8 The crayfish is an example of a crustacean. It has a carapace around the cephalothorax and the heart in the dorsal thorax area. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Subphylum Chelicerata includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders. This subphylum is predominantly terrestrial, although some marine species also exist. An estimated 103,0004 described species are included in subphylum Chelicerata.
The body of chelicerates may be divided into two parts and a distinct “head” is not always discernible. The phylum derives its name from the first pair of appendages: the chelicerae (Figure 3.9a), which are specialized mouthparts. The chelicerae are mostly used for feeding, but in spiders, they are typically modified to inject venom into their prey (Figure 3.9b). As in other members of Arthropoda, chelicerates also utilize an open circulatory system, with a tube-like heart that pumps blood into the large hemocoel that bathes the internal organs. Aquatic chelicerates utilize gill respiration, whereas terrestrial species use either tracheae or book lungs for gaseous exchange. What are the five subphyla of phylum arthropods?
Figure 3.9 (a) The chelicerae (first set of appendages, circled) are well developed in the Chelicerata, which includes scorpions (a) and spiders (b). Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51ef14f21b5eabd@10.8
3.3.3.4 Habitat and Adaptation
The arthropods are seen from 30,000 feet below to 20,000 feet above the sea level. These bilaterally symmetrical, jointed-leg invertebrates may be marine, fresh-water, terrestrial, subterranean and aerial. Some arthropods like barnacles are sedentary. Innumerable crustaceans which live as planktons move passively in the current of water. But welldeveloped structures are present in many arthropods for moving effectively by swimming, crawling and flying. Some arthropods within burrows, some are efficient diggers and many others build welldesigned nests. Certain arthropods like honey-bees, ants and termites are polymorphic and lead a complicated social life. All the food habits— herbivorous, carnivorous and omnivorous are seen among arthropods and various food-getting devices are met within this group.
Large numbers of arthropods live as parasites, and structural changes occur in them to adjust with the peculiar mode of life. Many arthropods are well- known for their habit of migration. Some of them can produce sound and nearly all are equipped with efficient sense organs. Some forms exhibit a phenomenon—suspended animation, to overcome unfavourable conditions. Sexual reproduction is often accompanied by courtship dances. The members may either be oviparous or viviparous or ovoviviparous and some forms exhibit parental care. Parthenogenesis is quite common in arthropods.
Arthropods are the largest animal phylum on earth. One million species of arthropods have been recognized worldwide. They show various types of adaptation to their environment. They are listed below.
1. Most arthropods are small in size.
2. Arthropods develop a prominent head, which is composed of
pairs of antennae and compound eyes. Arthropoda was the first group
of animals to develop a head.
3. The jointed appendages of arthropods occur in pairs. One or two
pairs of wings occur in aerial arthropods. This facilitates their
propagation.
4. The body of arthropods is covered with a chitinous exoskeleton.
Exoskeleton provides support to the body and sites for the
attachment of muscles. It also prevents water loss from the body.
The process of shedding the exoskeleton is called molting or ecdysis;
this facilitates the growth.
5. Arthropods have a complete digestive system with an anus and
mouth. Mouthparts of them are varied based on the type of diet
they get. Some of them have lapping and chewing, sucking or
siphoning
6. Breathing occurs through gills, trachea or book lungs.
7. The excretion of terrestrial arthropods occurs through Malpighian
tubules. Nitrogenous wastes are excreted as uric acid, reducing
the water loss from the body.
8. Arthropods are unisexual animals.
Self-Assessment Exercises 3
1. What is the most important unique feature of arthropods?
2. What is the meaning of bilateral symmetry as displayed by arthropods?
Summary
Flatworms are acoelomate organisms with three tissue layers. They lack circulatory and respiratory systems, and their excretory system is rudimentary. Their digestive system is often incomplete. There are four main classes: free-living turbellarians, ectoparasitic monogeneans, and endoparasitic trematodes and cestodes. Trematodes have complex life cycles with mollusk and vertebrate hosts, while cestodes infect vertebrate digestive systems.
Nematodes, within the Ecdysozoa clade, are pseudocoelomates with a complete digestive system. They include free-living and parasitic species, with diverse reproductive strategies. Their excretory system is underdeveloped, and their embryonic development involves external stages separated by molts.
Arthropods, the most successful animal phylum, exhibit segmented bodies and jointed appendages. Each body segment usually has a pair ofappendages. Arthropods have a chitinous exoskeleton and use gills, tracheae, or book lungs for respiration. They're classified based on mouthparts and appendage modifications. Their embryonic development often involves multiple larval stages.
UNIT 4 MOLLUSK AND ANNELIDS
Unit Structure
4.1 Introduction
4.2 Intended Learning Objectives (ILOs)
4.3 Main Contents
4.3.1 Phylum Mollusk
4.3.1.1 Characteristic Features of Mollusk
4.3.1.2 Mollusk Diversity
4.3.1.3 Habitat and Adaptation
4.3.2.1 Annelida
4.3.2.2 Characteristic Features of Annelida
4.3.2.3 Physiological Processes of Annelida
4.3.2.4 Annelid Diversity
4.3.2.5 Habitat and Adaptation
4.3.3 Similarities and differences between Mollusk and Annelid
4.4 Summary
4.5 Self-Assessment Exercises
4.1 Introduction
The mollusks are a diverse group (85,000 described species) of mostly marine species. They have a variety of forms, ranging from large predatory squid and octopus, some of which show a high degree of intelligence, to small grazing forms with elaborately sculpted and colored shells. The annelids traditionally include the oligochaetes, which include the earthworms and leeches, the polychaetes, which are a marine group, and two other smaller classes. The phyla Mollusca and Annelida belong to a clade called the Lophotrochozoa, which also includes the phylum Nemertea, or ribbon worms. They are distinct from the Ecdysozoa (nematodes and arthropods) based on evidence from analysis of their DNA, which has changed our views of the relationships among invertebrates.
4.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to:
1. Describe the unique anatomical features of mollusks
2. Describe the characteristic features of Mollusk
3. Describe the diversity in the mollusks
4.3.1 Phylum Mollusk
Mollusca is the predominant phylum in marine environments, where it is estimated that 23 percent of all known marine species belong to this phylum. It is the second most diverse phylum of animals with over 75,000 described species. The name “mollusca” signifies a soft body, as the earliest descriptions of mollusks came from observations of unshelled, soft-bodied cuttlefish (squid relatives). Although mollusk body forms vary, they share key characteristics, such as a ventral, muscular foot that is typically used for locomotion; the visceral mass, which contains most of the internal organs of the animal; and a dorsal mantle, which is a flap of tissue over the visceral mass that creates a space called the mantle cavity. The mantle may or may not secrete a shell of calcium carbonate. In addition, many mollusks have a scraping structure at the mouth, called a radula (Figure 4.1).The muscular foot varies in shape and function, depending on the type of mollusk (described below in the section on mollusk diversity). It is a retractable as well as extendable organ, used for locomotion and anchorage. Mollusks are eucoelomates, but the coelomic cavity is restricted to a cavity around the heart in adult animals. The mantle cavity, formed inside the mantle, develops independently of the coelomic cavity. It is a multi-purposespace, housing the gills, the anus, organs for sensing food particles in the water, and an outlet for gametes. Most mollusks have an open circulatory system with a heart that circulates the hemolymph in open spaces around the organs. The octopuses and squid are an exception to this and have a closed circulatory system with two hearts that move blood through the gills and a third, systemic heart that pumps blood through the rest of the body. What morphological feature gives the phylum Mollusca its name?4. Describe the features of an animal classified in phylum Annelida
Figure 4.1 There are many species and variations of mollusks; the gastropod mollusk anatomy is shown here, which shares many characteristics common with other groups. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
4.3.1.1 Characteristic Features of Mollusk
1. They are mostly marine in water, few freshwater and some terrestrial form.
2. They can be found inside other animals, as secret parasites.
3. They range in size from giant squids and clams to small snails, one mm long.
4. They have at least two radula and mantle characters, which are not found elsewhere.
5. The body is soft, unsegmented, bilateral symmetrical,
coelomates, triploblastic (except in Monoplacophora).
1. Body organization is tissue-systems grade.
1. The body has head, foot, mantle and visceral mass.
2. The body is covered with often ciliated one-layered epidermis.
3. The body is commonly protected by one or more pieces of exoskeletal calcareous shell secreted by the mantle.
4. Except in pelecypoda and scaphopoda, the head is distinct, containing the mouth , eyes, tentacles and other sense organs.
5. The ventral body is converted into a muscular plough-like surface, the foot which is modified in different ways for creeping, burrowing and swimming.
6. Mantle or pallium is a fold of a wall of the body that leaves the main body, mantle cavity, within itself.
7. Cavity of the body is hemocoel. The coelom is reduced and characterized by pericardial cavity, gonadial cavity.
8. Organ rasping, usually occurring radula or in pelecypoda.
9. Except in cephalopods, the circulatory system is open type.
10. There are numerous gills or ctenidia in the respiratory organs usually provided with osphradium at the base. In terrestrial forms the lung develops.
11. Respiration in Mollusca is provided by gills or lungs, or both.
12. Their respiratory pigments are haemocyanin;
13. Excretion is achieved by paired metanephridia (kidney).
1. The mollusca nervous system consists of paired prefrontal, pleural, pedal and visceral ganglia, along with longitudinal and transverse nervous connections. Usually ganglia form a circumentary ring.
1. Sense organs are composed of skin, statocysts and touch, smell, and taste receptors.Sexes are usually separate (dioecious) but some are monoecious (hermaphroditic)
1. Development through the trochophore stage called veliger larva is direct or with metamorphosis. 2. The visceral mass, in its compact form, contains the vital organs of the body, taking the form of dorsal humps or dome.
4.3.1.2 Mollusk Diversity
This phylum is comprised of seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. Class Aplacophora (“bearing no plates”) includes worm-like animals living mostly on deep ocean bottoms. These animals lack a shell but have aragonite spicules on their skin. Members of class Monoplacophora (“bearing one plate”) have a single, cap-like shell enclosing the body. The monoplacophorans were believed extinct and only known as fossils until the discovery of Neopilina galatheae in 1952. Today, scientists have identified nearly two dozen living species.
Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons” and bear an armor-like, eight-plated shell (Figure 4.2). These animals have a broad, ventral foot that is adapted for attachment to rocks and a mantle that extends beyond the shell in the form of a girdle. They breathe with ctenidia (gills) present ventrally. These animals have a radula modified for scraping. A single pair of nephridia for excretion is present.
Figure 4.2 This chiton from the class Polyplacophora has the eightplated shell indicative of its class. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Class Bivalvia (“two shells”) includes clams, oysters, mussels, scallops, and geoducks. They are found in marine and freshwater habitats. As the name suggests, bivalves are enclosed in a pair of shells (or valves) that are hinged at the dorsal side. The body is flattened on the sides. They feed by filtering particles from water and a radula is absent. They exchange gases using a pair of ctenidia, and excretion and osmoregulation are carried out by a pair of nephridia. In some species, the posterior edges of the mantle may fuse to form two siphons that inhale and exhale water. Some bivalves like oysters and mussels have the unique ability to secrete and deposit a calcareous nacre or “mother of pearl” around foreign particles that enter the mantle cavity. This property is commercially exploited to produce pearls.
Gastropods (“stomach foot”) include well-known mollusks like snails, slugs, conchs, sea hares, and sea butterflies. Gastropods include shellbearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell (Figure 4,3).
Figure 4.3 (a) Like many gastropods, this snail has a stomach foot and a coiled shell. (b) This slug, which is also a gastropod, lacks a shell. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51ef14f21b5eabd@10.8
The visceral mass in the shelled species is characteristically twisted and the foot is modified for crawling. Most gastropods bear a head with tentacles that support eyes. A complex radula is used to scrape food particles from the substrate. The mantle cavity encloses the ctenidia as well as a pair of nephridia. Are mollusks Metameric? Are molluscs metameric organisms?
The class Cephalopoda (“head foot” animals) includes octopuses, squids, cuttlefish, and nautilus. Cephalopods include shelled and reduced-shell groups. They display vivid coloration, typically seen in squids and octopuses, which is used for camouflage. The ability of some octopuses to rapidly adjust their colors to mimic a background pattern or to startle a predator is one of the more awe-inspiring feats of these animals. All animals in this class are predators and have beak-like jaws. All cephalopods have a well-developed nervous system, complex eyes, and a closed circulatory system. The foot is lobed and developed into tentacles and a funnel, which is used for locomotion. Suckers are present on the tentacles in octopuses and squid. Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart.
Cephalopods (Figure 4.3) are able to move quickly via jet propulsion by contracting the mantle cavity to forcefully eject a stream of water. Cephalopods have separate sexes, and the females of some species care for the eggs for an extended period of time. Although the shell is much reduced and internal in squid and cuttlefish, and absent altogether in octopus, nautilus live inside a spiral, multi-chambered shell that is filled with gas or water to regulate buoyancy.
Figure 4.4 The (a) nautilus, (b) giant cuttlefish, (c) reef squid, and (d) blue-ring octopus are all members of the class Cephalopoda. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Members of the class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells.” Tooth shells are open at both ends and usually lie buried in sand with the front opening exposed to water and the reduced head end projecting from the back of the shell. Tooth shells have a radula and a foot modified into tentacles, each with a bulbous end that catches and manipulates prey (Figure 4.5).
Figure 4.5 Antalis vulgaris shows the classic Dentaliidae shape that gives these animals their common name of “tusk shell.” Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
4.3.1.3Habitat and Adaptation
Phylum Mollusk is the second largest animal phylum on the planet. They are mostly marine, but many occur in fresh water and some even in damp soil. Molluscs have conquered most habitats in the ocean, with their distributions ranging from the intertidal zone, the open ocean to the deep sea and extreme environments like hydrothermal vents. They usually live in the sea shores or in shallow water, and some are pelagic and can sink down to the depth of about 35,000 feet. Most of the Molluscs are free-living forms.
Adaptation of a mollusk is the process of adjusting to different habitats and environmental conditions. Mollusks have adapted to all habitats except air, and have diverse modes of locomotion and feeding. Some of the unique adaptations of mollusks include a specialized feeding organ called a radula, a dorsal layer of tissue called a mantle, and a complex nervous system. Some mollusks also have highly developed eyes, ink glands, and skin cells that can change color. Among the adaptations Mollusks have gained over time, the most obvious, and most important, is the muscular foot all mollusks possess. This foot adaptation is also something that differentiates all different species of Mollusks. There is a lot of variation in this adaptation, so species to species, Mollusk feet will look different. Are mussels Gastropoda? The freshwater mollusks include two classes, the Gastropoda (snails and limpets) and the Bivalvia (clams and mussels). The gastropods constitute the most diverse class of the phylum Mollusca, with about 75,000 species of marine and freshwater snails worldwide.
Self-Assessment Exercises 1
1. What is the purpose of a radula?
2. How do Gastropods move?
4.3.2 Annelids
Phylum Annelida are segmented worms found in marine, terrestrial, and freshwater habitats, but the presence of water or humidity is a critical factor for their survival in terrestrial habitats. The name of the phylum is derived from the Latin word annellus, which means a small ring. Approximately 16,500 species have been described. The phylum includes earthworms, polychaete worms, and leeches. Like mollusks, annelids exhibit protostomic development. Annelids are bilaterally symmetrical and have a worm-like appearance. Their particular segmented body plan results in repetition of internal and external features in each body segment. This type of body plan is called metamerism. The evolutionary benefit of such a body plan is thought to be the capacity it allows for the evolution of independent modifications in different segments that perform different functions. The overall body can then be divided into head, body, and tail.
4.3.2.1Characteristic of Phylum Annelids
1. They are mostly aquatic, some are terrestrial.
2. They are generally burning animals, some are sedentary or free living, and some are ectoparasites.
3. The body is vermiform, bilaterally symmetrical, and metamerically segmented.
4. They have straight tube alimentary canal, and undergo extracellular digestion.
5. Has segmentally arranged Locomotory organs, repeated groups of chitinous setae or chaetae.
6. Leaches have no setae.
1. Respiration is generally through body surface or through a special projection of parapods.
2. Has well developed closed type blood vascular system.
3. Possesses Nephridia which is the excretory organs.
4. Nervous system consists of paired cerebral ganglia or brain, a double ventral nerve cord bearing segmental ganglia.
5. Gonads develop from the coelomic epithelium.
6. Sex may be separate or united, and development may be direct or indirect.
4.3.2.2Physiological Processes of Annelids
The skin of annelids is protected by a cuticle that is thinner than the cuticle of the ecdysozoans and does not need to be molted for growth. Chitinous hairlike extensions, anchored in the skin and projecting from the cuticle, called chaetae, are present in every segment in most groups. The chaetae are a defining character of annelids. Polychaete worms have paired, unjointed limbs called parapodia on each segment used for locomotion and breathing. Beneath the cuticle there are two layers of muscle, one running around its circumference (circular) and one running the length of the worm (longitudinal). Annelids have a true coelom in which organs are distributed and bathed in coelomic fluid. Annelids possess a well-developed complete digestive system with specialized organs: mouth, muscular pharynx, esophagus, and crop. A crosssectional view of a body segment of an earthworm is shown in Figure 4.6; each segment is limited by a membrane that divides the body cavity into compartments.
Annelids have a closed circulatory system with muscular pumping “hearts” in the anterior segments, dorsal and ventral blood vessels that run the length of the body with connections in each segment, and capillaries that service individual tissues. Gas exchange occurs across the moist body surface. Excretion is carried out by pairs of primitive “kidneys” called metanephridia that consist of a convoluted tubule and an open, ciliated funnel present in every segment. Annelids have a welldeveloped nervous system with two ventral nerve cords and a nerve ring of fused ganglia present around the pharynx. How does digestion in organisms of the phylum Annelida take place?
What type of digestive system do they have
Figure 4.6 Extracellular digestion in annelids. In this schematic showing the basic anatomy of annelids, the digestive system is indicated in green, the nervous system is indicated in yellow, and the circulatory system is indicated in red. Source: http://cnx.org/contents/185cbf87-c72e-48f5- b51e-f14f21b5eabd@10.8
Annelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads (as in polychaetes). How does a change in the circulatory system organization support the body designs in cephalopods compared to other mollusks?
Self-Assessment Exercises 2
1. Describe the morphology and anatomy of mollusks.
2. What are the anatomical differences between nemertines and mollusks?
Top of Form
Bottom of Form
4.3.2.3Annelid Diversity
The phylum Annelida is divided into three classes: oligochaetes (for example, earthworms), hirudineans (such as leeches) and polychaetes (these are mostly marine aquatic organisms with parapodia, such as nereis). The phylum includes the classes Polychaeta and Clitellata (Figure 4.67); the latter contains subclasses Oligochaeta, Hirudinoidea, and Branchiobdellida.
Earthworms are the most abundant members of the subclass Oligochaeta, distinguished by the presence of the clitellum, a ring structure in the skin that secretes mucus to bind mating individuals and forms a protective cocoon for the eggs. They also have a few, reduced chaetae (oligo- = “few”; -chaetae = “hairs”). The number and size of chaetae is greatly diminished in oligochaetes as compared to the polychaetes (poly- = “many”; -chaetae = “hairs”). The chaetae of polychaetes are also arranged within fleshy, flat, paired appendages on each segment called parapodia. The subclass Hirudinoidea includes leeches. Significant differences between leeches and other annelids include the development of suckers at the anterior and posterior ends, and the absence of chaetae. Additionally, the segmentation of the body wall may not correspond to internal segmentation of the coelomic cavity. This adaptation may allow leeches to swell when ingesting blood from host vertebrates. The subclass Branchiobdellida includes about 150 species that show similarity to leeches as well as oligochaetes. All species are obligate symbionts, meaning that they can only survive associated with their host, mainly with freshwater crayfish. They feed on the algae that grows on the carapace of the crayfish. Into which classes is the phylum Annelida divided?
Figure 4.7 The (a) earthworm and (b) leech are both annelids. Source: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
4.3.2.4Habitat and Adaptation
Annelids are found worldwide in nearly every habitat on the planet, but they prefer to live in wet environments due to their inability to protect themselves from desiccation. They are especially common in oceanic waters, fresh waters, and damp soils. Most polychaetes live in the ocean, where they either float, burrow, wander on the bottom, or live in tubes they construct.
Annelids have adapted to different environments through convergent evolution. Some annelids adapted to the space between sand grains by progenesis, while others evolved by miniaturization of ancestral adult stages. Miniaturization is as important as progenesis in the adaptation to the interstitium. When the environmental conditions in an earthworm’s habitat change, many earthworms become inactive in a process called aestivation. They move deeper into the soil, coil into a tight ball, excrete a protective mucus and lower their metabolic rate in order to reduce water loss. Species lacking a pelagic trochophore stage show special adaptive features.
4.3.3 Similarities between Mollusk and Annelids
Mollusks and annelids are very similar creatures. Both groups are invertebrates, meaning they lack a backbone. They also both have a soft, unsegmented body. Mollusks and annelids also both have a coelom, or a fluid-filled body cavity. Another similarity between these two groups is that they both have a muscular foot used for locomotion.
Self-Assessment Exercises 3
1. What is the purpose of chromatophores?
2. What is the difference between annelids and molluscs?
4.4 Summary
The phylum Mollusca is a large, mainly marine group of invertebrates. Mollusks show a variety of morphologies. Many mollusks secrete a calcareous shell for protection, but in other species, the shell is reduced or absent. Mollusks are protostomes. The dorsal epidermis in mollusks is modified to form the mantle, which encloses the mantle cavity and visceral organs. This cavity is distinct from the coelomic cavity, which the adult animal retains, surrounding the heart. Respiration is facilitated by gills known as ctenidia. A chitinous scraper called the radula is present in most mollusks. Mollusks are mostly dioecious and are divided into seven classes.
The phylum Annelida includes worm-like, segmented animals. Segmentation is both external and internal, which is called metamerism. Annelids are protostomes. The presence of chitinous hairs called chaetae is characteristic of most members. These animals have well-developed nervous and digestive systems. Polychaete annelids have parapodia that participate in locomotion and respiration. Suckers are seen in the order Hirudinea. Breeding systems include separate sexes and hermaphroditism.
Unit 5 Echinoderms and Chordates
Unit Structure
5.1 Introduction
5.2 Intended Learning Outcomes (ILOs)
5.3.1 Echinoderms
5.3.1.1 Characteristic Features of Echinoderms
5.3.1.2 Physiological Processes of Echinoderms
5.3.2 Echinoderm Diversity
5.3.2.1 Habitat
5.3.2.2 Ecological Adaptation
5.3.3 Chordates
5.3.3.1 Invertebrate Chordates
5.3.3.2 Habitat and Ecological Adaptation
5.3.3.3 Vertebrates and Invertebrates Animals
5.4 Summary
5.1 Introduction
Echinoderms and chordates are two closely-related groups of animals. They belong to the clade Deuterostomia, which means their blastopore develops into the anus. Deuterostomes include the phyla Echinodermata and Chordata (which includes the vertebrates) and two smaller phyla. Deuterostomes share similar patterns of early development. They also show bilateral symmetry at some stage of their life cycle and exhibit radial cleavage. Echinoderms are marine organisms that include starfish, while chordates include humans and other vertebrates. Their shared common ancestor was likely a bilaterally symmetrical organism with a cephalized nervous system.
5.2 Intended Learning Objectives (ILOs)
By the end of this section, you will be able to:
1. Describe the distinguishing characteristics of echinoderms
2. Explain the characteristic features of echinoderms
3. Physiological Processes of Echinoderms
4. Describe the distinguishing characteristics of chordates
5.3.1 Echinoderms
Echinodermata are named for their spiny skin (from the Greek “echinos” meaning “spiny” and “dermos” meaning “skin”). The phylum includes about 7,000 described living species, such as sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars. Echinodermata are exclusively marine. Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles (Figure 5.1), although the early larval stages of all echinoderms have bilateral symmetry. The endoskeleton is developed by epidermal cells, which may also possess pigment cells, giving vivid colors to these animals, as well as cells laden with toxins. These animals have a true coelom, a portion of which is modified into a unique circulatory system called a water vascular system. An interesting feature of these animals is their power to regenerate, even when over 75 percent of their body mass is lost.
5.3.1.1Characteristic Features of Phylum Echinodermata
i. All Echinoderms are marine animals, and the adults are mostly with Pentamerous radial symmetry (ie can be divided into 5 equal parts). Body is not metamerically segmented;
ii. Could be rounded, cylindrical or star shaped without head.
iii. They have no brain, only few specialized sense organs.
iv. They have a complete digestive system.
v. Locomotive is mainly by tube feet, in some by means of spines or by movement of arms.
vi. They have no olfactory organs.
vii. Sexes are separate, fertilization is external.
viii. They have indeterminate type of development.
ix. Respiration is by dermal branchiae, tube feet or respiratory trees.
x. They have an internal skeleton (endoskeleton) made up of plates of calcium carbonate, imbedded in the body wall.
5.3.1.2Physiological Processes of Echinoderms
Echinoderms have a unique system for gas exchange, nutrient circulation, and locomotion called the water vascular system. The system consists of a central ring canal and radial canals extending along each arm. Water circulates through these structures allowing for gas, nutrient, and waste exchange. A structure on top of the body, called the madreporite, regulates the amount of water in the water vascular system. “Tube feet,” which protrude through openings in the endoskeleton, may be expanded or contracted using the hydrostatic pressure in the system. The system allows for slow movement, but a great deal of power, as witnessed when the tube feet latch on to opposite halves of a bivalve mollusk, like a clam, and slowly, but surely pull the shells apart, exposing the flesh within.
Figure 5.1 The echinoderm nervous system has a nerve ring at the center and five radial nerves extending outward along the arms. There is no centralized nervous control. Echinoderms have separate sexes and release their gametes into the water where fertilization takes place. Source: http://www.ucmp.berkeley.edu/echinodermata/blastoidea.html Echinoderms may also reproduce asexually through regeneration from body parts. What is the unique system for gas exchange, nutrient circulation, and locomotion in Echinoderms?
Self-Assessment Exercises 1
1. Why are the adult Echinoderms said to have a mostly Pentamerous radial symmetry?
2. What is the structure on top of the body,that regulates the amount of water in the water vascular system?
5.3.2 Echinoderm Diversity
This phylum is divided into five classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers) (Figure 5.2). Perhaps the best-known echinoderms are members of the class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1,800 species known. The characteristics of sea stars that set them apart from other echinoderm classes include thick arms that extend from a central disk where organs penetrate into the arms. Sea stars use their tube feet not only for gripping surfaces but also for grasping prey. Sea stars have two stomachs, one of which they can evert through their mouths to secrete digestive juices into or onto prey before ingestion. This process can essentially liquefy the prey and make digestion easier.
Brittle stars have long, thin arms that do not contain any organs. Sea urchins and sand dollars do not have arms but are hemispherical or flattened with five rows of tube feet, which help them in slow movement. Sea lilies and feather stars are stalked suspension feeders. Sea cucumbers are soft-bodied and elongate with five rows of tube feet and a series of tube feet around the mouth that are modified into tentacles used in feeding. Which is the best-known class of echinoderms?
Figure 5.2 Different members of Echinodermata include the (a) sea star in class Asteroidea, (b) the brittle star in class Ophiuroidea, (c) the sea urchins of class Echinoidea, (d) the sea lilies belonging to class Crinoidea, and (e) sea cucumbers representing class Holothuroidea. Source: http://www.ucmp.berkeley.edu/echinodermata/blastoidea.html
5.3.2.1 Habitat
Echinoderms are globally distributed in almost all depths, latitudes and environments in the ocean. Adults are mainly benthic, living on the seabed, whereas larvae are often pelagic, living as plankton in the open ocean. Some holothuroid adults such as Pelagothuria are however pelagic. Some crinoids are pseudo-planktonic, attaching themselves to floating logs and debris, although this behaviour was exercised most extensively in the Paleozoic, before competition from organisms such as barnacles restricted the extent of the behaviour.
5.3.2.2Ecological Adaptation
Echinoderms have several adaptations to their environment. Some of these adaptations include:
1. Radial symmetry and five lines of symmetry
2. Amazing vision or about 20/20 vision
3. A nerve ring at the center and five radial nerves extending outward along the arms
4. No centralized nervous control
5. Separate sexes and release of gametes into the water where fertilization takes place
6. Asexual reproduction through regeneration from body parts
7. Defense mechanisms such as spines, toxins, and camouflage
8. Water vascular system
9. Body regeneration
10. Cutaneous and cloacal respiration
11. Open circulatory systems
Self-Assessment Exercises 2
1. What are the five classes of Echinoderms?
2. Which members of Echinoderms have two stomachs, which confers on it the ability to liquefy its prey and make digestion easier?
5.3.3 Chordates
The majority of species in the phylum Chordata are found in the subphylum Vertebrata, which include many species with which we are familiar. The vertebrates contain more than 60,000 described species, divided into major groupings of the lampreys, fishes, amphibians, reptiles, birds, and mammals. Animals in the phylum Chordata share four key features that appear at some stage of their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail (Figure 5.3). In certain groups, some of these traits are present only during embryonic development. The chordates are named for the notochord, which is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates and in the adult stage of some chordate species. It is located between the digestive tube and the nerve cord, and provides skeletal support through the length of the body. In some chordates, the notochord acts as the primary axial support of the body throughout the animal’s lifetime. In vertebrates, the notochord is 1. What are the five classes of Echinoderms? 2. Which members of Echinoderms have two stomachs, which confers on it the ability to liquefy its prey and make digestion easier?
present during embryonic development, at which time it induces the development of the neural tube and serves as a support for the developing embryonic body. The notochord, however, is not found in the postnatal stage of vertebrates; at this point, it has been replaced by the vertebral column (the spine). What happens to the notochord in vertebrates and protochordates? In vertebrates, the notochord disappears and to produce the spine (vertebral column). In protochordates, the notochord remains during their whole life.
The dorsal hollow nerve cord is derived from ectoderm that sinks below the surface of the skin and rolls into a hollow tube during development. In chordates, it is located dorsally to the notochord. In contrast, other animal phyla possess solid nerve cords that are located either ventrally or laterally. The nerve cord found in most chordate embryos develops into the brain and spinal cord, which compose the central nervous system.
Pharyngeal slits are openings in the pharynx, the region just posterior to the mouth, that extend to the outside environment. In organisms that live in aquatic environments, pharyngeal slits allow for the exit of water that enters the mouth during feeding. Some invertebrate chordates use the pharyngeal slits to filter food from the water that enters the mouth. In fishes, the pharyngeal slits are modified into gill supports, and in jawed fishes, jaw supports. In tetrapods, the slits are further modified into components of the ear and tonsils, since there is no longer any need for gill supports in these air-breathing animals. Tetrapod means “fourfooted,” and this group includes amphibians, reptiles, birds, and mammals. (Birds are considered tetrapods because they evolved from tetrapod ancestors.)
The post-anal tail is a posterior elongation of the body extending beyond the anus. The tail contains skeletal elements and muscles, which provide a source of locomotion in aquatic species, such as fishes. In some terrestrial vertebrates, the tail may also function in balance, locomotion, courting, and signaling when danger is near. In many species, the tail is absent or reduced; for example, in apes, including humans, it is present in the embryo, but reduced in size and nonfunctional in adults. What are the characteristic features of the chordates? What are the two main subdivisions of the phylum Chordat
Figure 5.3 Chordate features. In chordates, four common features appear at some point during development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. The endostyle is embedded in the floor of the pharynx. Source: http://www.environment.gov.au/biodiversity/abrs/publications/other/spe cies-numbers/2009/03-exec-summary.html.
5.3.3.1Invertebrate
Chordates In addition to the vertebrates, the phylum Chordata contains two clades ofinvertebrates: Urochordata (tunicates) and Cephalochordata (lancelets). Members of these groups possess the four distinctive features of chordates at some point during their development.The tunicates (Figure 5.4) are also called sea squirts. The name tunicate derives from the cellulose-like carbohydrate material, called the tunic, which covers the outer body. Although tunicates are classified as chordates, the adult forms are much modified in body plan and do not have a notochord, a dorsal hollow nerve cord, or a post-anal tail, although they do have pharyngeal slits. The larval form possesses all four structures. Most tunicates are hermaphrodites. Tunicate larvae hatch from eggs inside the adult tunicate’s body. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on which it can attach, usually in a dark or shaded location. It then attaches by the head to the substrate and undergoes metamorphosis into the adult form, at which point the notochord, nerve cord, and tail disappear.
Figure 5.4 (a) This photograph shows a colony of the tunicate Botrylloides violaceus. In the (b) larval stage, the tunicate can swim freely until it attaches to a substrate to become (c) an adult. Source: http://www.environment.gov.au/biodiversity/abrs/publications/other/spe cies-numbers/2009/03-exec-summary.html
Most tunicates live a sessile existence in shallow ocean waters and are suspension feeders. The primary foods of tunicates are plankton and detritus. Seawater enters the tunicate’s body through its incurrent siphon. Suspended material is filtered out of this water by a mucus net (pharyngeal slits) and is passed into the intestine through the action of cilia. The anus empties into the excurrent siphon, which expels wastes and water.
Lancelets possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage (Figure 5.5). The notochord extends into the head, which gives the subphylum its name (Cephalochordata). Extinct fossils of this subphylum date to the middle of the Cambrian period (540–488 mya).The living forms, the lancelets, are named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders. What are two clades of invertebrates contained in the phylum Chordata?
Figure 5.5 Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Source: http://www.environment.gov.au/biodiversity/abrs/publications/other/spe cies-numbers/2009/03-exec-summary.html.
5.3.3.2 Habitat and Ecological Adaptation
These organisms reside in marine environments living individually or in colonies. Invertebrate chordates feed on tiny organic matter, such as plankton, suspended in the water. Invertebrate chordates are coelomates or animals with a true body cavity.
An animal may adapt to its habitat in different ways. It may be a physical or structural adaptation, just as the limbs of birds have modified into wings or the way the cheetah is shaped for running at a fast speed. It may be in the way the body works in circulating and respiration, for instance the gills that fish have enable them to breathe in water. Or it may be the way the animal behaves whether it is hunting for food, or running fast to avoid predators or migrating to other places for food or survival. An animal's environment consists of many different things. The climate, the kinds of food plants that grow in it, other animals that may be predators or competitors- the animal must learn to adapt to each of these factors in order to survive. With increasing population growth and human activity that disturbs the natural habitat, animals must learn to adapt to these kinds of threats as well.
Animals in the wild can only live in places they are adapted to. They must have the right kind of habitat where they can find the food and space they need. Did you know that animals camouflage themselves so they can adapt to their environment? Adaptation can protect animals from predators or from harsh weather. Many birds can hide in the tall grass and weeds and insects can change their colour to blend into the surroundings. This makes it difficult for predators to seek them out for food.
Some animals, like the apple snail, can survive in different ecosystemsfrom swamps, ditches and ponds to lakes and rivers. It has a lung/gills combination that reflects its adaptation to habitats with oxygen poor water. This is often the case in swamps and shallow waters. In the harsh cold climate of Alaska, the animals have learnt to adapt to the weather by storing food in their body and protecting themselves from the cold with thick furs. Human inhabitants in Alaska have also learnt to cope with the environment by building shelters that insulate and hold the heat, and yet do not allow the structure to melt.
5.3.3.3Vertebrates and Invertebrates Animals
Vertebrates are typically more complex as compared to invertebrates, with a more developed nervous system and a greater level of cognitive function. They have an integral skeleton made up of bones and cartilage that provide support and protection for their internal organs. They also have a wide range of reproductive strategies, including internal and external fertilization. Coming to invertebrates, on the other hand, there is a lack of a true skeleton and instead, have an exoskeleton or a hydrostatic skeleton. They are generally less complex than vertebrates, having a simpler nervous system. They are also found in a variety of habitats, but many are restricted to specific environments. Which of vertebrates or invertebrates, are typically more complex organisms?
Self-Assessment Exercises 3
1. Sessile adult tunicates lose the notochord; what does this suggest about the function of this structure?
2. During embryonic development, what features do we share with tunicates or lancelets?
5.4 Summary
Echinoderms are deuterostome marine organisms. This phylum of animals bear a calcareous endoskeleton composed of ossicles covered by a spiny skin. Echinoderms possess a water-based circulatory system. The madreporite is the point of entry and exit for water for the water vascular system. The characteristic features of Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates. Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Unit 6 Vertebrates I: Fishes and Amphibians
Unit Structure
6.1 Introduction
6.2 Intended Learning Outcomes (ILOs)
6.3 Main Contents
6.3.1 Vertebrate organisms
6.3.1.1 Fishes
6.3.1.2 Jawless Fishes
6.3.1.3 Jawed Fishes
6.3.1.4 Bony Fishes
6.3.2 Amphibians
6.3.2.1 Characteristics Features of Amphibians
6.3.2.2 Amphibian Diversity
6.3.2.3 Ecological Adaptation of Amphibians
6.4 Summary
Introduction
Vertebrates are among the most recognizable organisms of the animal kingdom. More than 62,000 vertebrate species have been identified. The vertebrate species now living represent only a small portion of the vertebrates that have existed. The best-known extinct vertebrates are the dinosaurs, a unique group of reptiles, reaching sizes not seen before or since in terrestrial animals. They were the dominant terrestrial animals for 150 million years, until they died out near the end of the Cretaceous period in a mass extinction. A great deal is known about the anatomy of the dinosaurs, given the preservation of their skeletal elements in the fossil record.
Intended Learning Objectives (ILOs)
By the end of this section, you will be able to:
1. Describe the general characteristics and classification of vertebrates
2. Describe the difference between jawless and jawed fishes
Vertebrate Organisms
Vertebrates have several basic features. First, vertebrates have a complex central nervous system. This includes a well-developed brain and a spinal cord that is held in the vertebral column. Vertebrates also have a closed circulatory system. This means the blood of a vertebrate circulates through its body in blood vessels. Vertebrates also have skin that is protected by hair, feathers, or scales. Finally, as embryos, vertebrates have neural crest cells. These cells develop into nerve cells and cells that form facial features.
In addition to these basic features, there are five key characteristics that are common to all vertebrates. These include:
1. Vertebra/backbone: Vertebra are a series of small bones that make up the backbone. The spinal cord passes through the vertebral column.
2. Skull: Vertebrates have a skull. This protects the delicate, welldeveloped brain.
3. Endoskeleton: Vertebrates have a well-developed endoskeleton.
4. This is an internal skeleton that provides structure to the vertebrate's body.
5. Bilateral Symmetry: These organisms are bilaterally symmetrical. This means that vertebrates have mirror-image right and left halves of their bodies.
6. Two Pairs of Appendages: Vertebrates have appendages such as wings, fins, or limbs. These appendages are seen in pairs on the vertebrate's body.
Vertebrates are divided into five classes: - Pisces - Amphibia - Reptilia - Aves, and - Mammalia. What does the complex central nervous system of vertebrates consisted? Vertebrates have a complex central nervous system that includes a well-developed brain and a spinal cord that is held in the vertebral column.
Self-Assessment Exercises 1
1. What are the five classes of Vertebrates?
2. What are the two pairs of appendages possessed by vertebrates?
3.3.1.1 Fishes
Modern fishes include an estimated 31,000 species. Fishes were the earliest vertebrates, and jawless fishes were the earliest of these. Jawless fishes—the present day hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, distinguishing them from the invertebrate chordates. The jawed fishes evolved later and are extraordinarily diverse today. Fishes are active feeders, rather than sessile, suspension feeders.
3.3.1.2 Jawless Fishes
Jawless fishes are craniates (which includes all the chordate groups except the tunicates and lancelets) that represent an ancient vertebrate lineage that arose over one half-billion years ago. Some of the earliest jawless fishes were the ostracoderms (which translates as “shell-skin”). Ostracoderms, now extinct, were vertebrate fishes encased in bony armor, unlike present-day jawless fishes, which lack bone in their scales.
The clade Myxini includes 67 species of hagfishes. Hagfishes are eellike scavengers that live on the ocean floor and feed on dead invertebrates, other fishes, and marine mammals (Figure 6.1a). Hagfishes are entirely marine and are found in oceans around the world except for the polar regions. A unique feature of these animals is the slime glands beneath the skin that are able to release an extraordinary amount of mucus through surface pores. This mucus may allow the hagfish to escape from the grip of predators. Hagfish are known to enter the bodies of dead or dying organisms to devour them from the inside.
Figure 6.1 (a) Pacific hagfishes are scavengers that live on the ocean floor. (b) These parasitic sea lampreys attach to their lake trout host by suction and use their rough tongues to rasp away flesh in order to feed on the trout’s blood. Source: https://openstax.org/books/conceptsbiology/pages/15-6-vertebra
The skeleton of a hagfish is composed of cartilage, which includes a cartilaginous notochord, which runs the length of the body, and a skull. This notochord provides support to the fish’s body. Although they are craniates, hagfishes are not vertebrates, since they do not replace the notochord with a vertebral column during development, as do the vertebrates.
The clade Petromyzontidae includes approximately 40 species of lampreys. Lampreys are similar to hagfishes in size and shape; however, lampreys have a brain case and incomplete vertebrae. Lampreys lack paired appendages and bone, as do the hagfishes. As adults, lampreys are characterized by a toothed, funnel-like sucking mouth. Some species are parasitic as adults, attaching to and feeding on the body fluids of fish (Figure 15.37b). Most species are free-living.
Lampreys live primarily in coastal and fresh waters and have a worldwide temperate region distribution. All species spawn in fresh waters. Eggs are fertilized externally, and the larvae are distinctly different from the adult form, spending 3 to 15 years as suspension feeders. Once they attain sexual maturity, the adults reproduce and die within days. Lampreys have a notochord as adults.
6.3.1.3 Jawed Fishes
Gnathostomes or “jaw-mouths” are vertebrates that have jaws and include both cartilaginous and bony fishes. One of the most significant developments in early vertebrate evolution was the origin of the jaw, which is a hinged structure attached to the cranium that allows an animal to grasp and tear its food. The evolution of jaws allowed early gnathostomes to exploit food resources that were unavailable to jawless fishes.
The clade Chondrichthyes, the cartilaginous fishes, is diverse, consisting of sharks (Figure 6.3a), rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras, or ghost sharks. Chondrichthyes have paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the middle Devonian. They are thought to have descended from an extinct group that had a skeleton made of bone; thus, the cartilaginous skeleton of Chondrichthyes is a later development. Parts of the shark skeleton are strengthened by granules of calcium carbonate, but this is not the same as bone.
Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for some or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Shark teeth likely evolved from the jagged scales that cover their skin. Some species of sharks and rays are suspension feeders that feed on plankton
Figure 6.3(a) This hammerhead shark is an example of a predatory cartilaginous fish. (b) This stingray blends into the sandy bottom of the ocean floor when it is feeding or awaiting prey. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and electroreception, the latter being perhaps the most sensitive of any animal. Organs called ampullae of Lorenzini allow sharks to detect the electromagnetic fields that are produced by all living things, including their prey. Electroreception has only been observed in aquatic or amphibious animals. Sharks, together with most fishes, also have a sense organ called the lateral line, which is used to detect movement and vibration in the surrounding water, and a sense that is often considered homologous to “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of the fish’s body.
Sharks reproduce sexually and eggs are fertilized internally. Most species are ovoviviparous, that is, the fertilized egg is retained in the oviduct of the mother’s body, and the embryo is nourished by the egg yolk. The eggs hatch in the uterus and young are born alive and fully functional. Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body. Embryos are protected by a shark egg case or “mermaid’s purse” that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and give the newborn shark cover. A few species of sharks are viviparous, that is, the young develop within the mother’s body, and she gives live birth.
Rays and skates include more than 500 species and are closely related to sharks. They can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface (Figure 6.3b). Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution
6.3.1.4 Bony Fishes
Members of the clade Osteichthyes, or bony fishes, are characterized by a bony skeleton. The vast majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today. Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. This characteristic has only reverted in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered in overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects vibrations in water. Unlike sharks, some bony fish depend on their eyesight to locate prey. Bony fish are also unusual in possessing taste cells in the head and trunk region of the body that allow them to detect extremely small concentrations of molecules in the water. All bony fishes, like the cartilaginous fishes, use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum. Unlike sharks, bony fishes have a swim bladder, a gas-filled organ that helps to control the buoyancy of the fish. Bony fishes are further divided into two clades with living members: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). The ray-finned fishes include many familiar fishes—tuna, bass, trout, and salmon (Figure 6.4a), among others. Ray-finned fishes are named for the form of their fins— webs of skin supported by bony spines called rays. In contrast, the fins of lobe-finned fishes are fleshy and supported by bone (Figure 6.4b). Living members of lobe-finned fishes include the less familiar lungfishes and coelacanth. IN-TEXT QUESTION (ITQ): Which of the fishes were the earliest vertebrates that evolved? Fishes were the earliest vertebrates, and jawless fishes were the earliest of these.
Figure 6.4 The (a) sockeye salmon and (b) coelacanth are both bony fishes of the Osteichthyes clade. The coelacanth, sometimes called a lobe-finned fish, was thought to have gone extinct in the Late Cretaceous period 100 million years ago until one was discovered in 1938 between Africa and Madagascar. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Self-Assessment Exercises 2
1. What are jawless fishes?
2. What is the mot important characteristic of members of the clade Osteichthyes?
6.3.2 Amphibians
Amphibians are vertebrate tetrapods. Amphibia includes frogs, salamanders, and caecilians. The term amphibian means “dual life,” which is a reference to the metamorphosis that many frogs undergo from a tadpole to an adult and the mixture of aquatic and terrestrial environments in their life cycle. Amphibians evolved in the Devonian period and were the earliest terrestrial tetrapods. As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of salamanders and all caecilians possess only vestigial limbs. An important characteristic of extant amphibians is a moist, permeable skin, achieved by mucus glands. The moist skin allows oxygen and carbon dioxide exchange with the environment, a process called cutaneous respiration. All living adult amphibian species are carnivorous, and some terrestrial amphibians have a sticky tongue that is used to capture prey. 90% of all amphibian species are frogs. IN-TEXT QUESTION (ITQ): Which chordate class is considered evidence of the transition of vertebrates from an aquatic environment to dry land?
Amphibians are totally aquatic during their larval stage and partially terrestrial animals as adults. Because of this, they are considered intermediate organisms in the evolutionary passage of vertebrates from an aquatic to dry land. Amphibians are also the first tetrapod animals; that is, the first with two pairs of limbs, a typical feature of terrestrial vertebrates. The name “amphibian” comes from the double life (aquatic as larvae and partially terrestrial as adults) of these animals.
6.3.2.1 Characteristics Features of Amphibians
Below are some of the characteristics shared by the amphibians.
1. Egg Are Fertilized Outside of the Body
Most amphibians reproduce in fresh water while a few lay their eggs on land and have developed mechanisms to keep the eggs moist. Reproduction in amphibians has more similarities with the fish than with reptiles or mammals. Although they reproduce sexually, fertilization and development of the young ones take place outside the body.
Cold-Blooded
Although amphibians exhibit both terrestrial and aquatic characteristic, they are strictly cold-blooded or ectothermic. They do not have the internal mechanisms to regulate their own body temperatures like mammals do. They rely on the external environment to regulate their body temperature. Amphibians tend to bask in the sun to raise their body temperature and retreat to a cold place to lower their temperature. Their cold-blooded nature has limited the ecosystem in which they can thrive in since they cannot survive in areas of high or low temperatures. Amphibians do not have hair or fur to insulate them from heat loss. To survive the cold winter, most amphibians remain active throughout the period. Some also sink deep into the water to stay warm. Other species hibernate at the bottom of the ponds. Salamanders have the ability to antifreeze ice forming around them by converting glycogen into glucose.
3. Breathe Through Skin
Amphibians have primitive lungs compared to other amniotes. They possess large alveoli and few internal septa, responsible for a slow oxygen diffusion rate into the blood. The lungs have low internal volumes and cannot process as much air as mammals or reptiles. Some species of salamanders are lungless and have to employ other means to breathe. Most amphibians exchange gases or breathe through their moist, permeable skin. To facilitate sufficient gaseous exchange, the vascular skin of the amphibians must be moist. The moist skin allows the oxygen to diffuse at a sufficiently high rate. The process by which gaseous exchange takes place through the skin is called cutaneous respiration. Aquatic amphibians like the Titicaca water frog can rely entirely on cutaneous respiration since the concentration of oxygen in water increases at both low temperature and high rate of flow. A network of cutaneous capillaries enables for the exchange of gases and the diffusion of water and iron between the environment and the animal.
4. Carnivores
Amphibians are mainly carnivores and feed on almost anything that moves and they can swallow. The adult amphibian is a predator with its diet consisting of a wide variety of food. Some of these foods include spiders, earthworms, beetles, and caterpillars. Burrowing caecilians mainly feed on earthworms whereas salamanders and anurans feed mainly on insects and arthropods. Large amphibians can also feed on small vertebrates such as birds and mammals. Food is often selected by sight, even in areas with dim light.
5. Time Spent In Water And On Land
Amphibians spend their life both on land and in water. The term “amphibian” is a Greek word for “amphibious” which means “living a double life.” Most have a biphasic life cycle which involves the fertilization and development of eggs and larvae in water. The larvae metamorphose into a semi-terrestrial or terrestrial juvenile and adults. On the evolutionary tree, amphibians are found midway between fish which fully live in water and reptiles and mammals which lead a fully terrestrial lifestyle. Adult amphibians have to live near water since they need steady moisture supply in order to survive. They can be found in a wide range of habitat near water including swamps, streams, forests, and dump areas.
6.3.2.2Amphibian Diversity
Amphibians are animals that are characterized by their ability to survive both in water and on land. The name “amphibian” is derived from the Greek word “amphibious” which means “to live a double life.” There are over 6,500 living species of amphibians with the majority of the species living within fresh aquatic water ecosystem. Most of them are born in water and start off as a larva and develop a land-based lifestyle as they develop. The class Amphibia is divided into three modern orders:
1. Anura (“tail-less ones”), which includes the toads and frogs.
2. Apoda (“legless ones”), which comprises the caecilians.
3. Urodela (“tailed-ones”), which are mainly salamanders.
Living salamanders (Figure 6.5a) include approximately 500 species, some of which are aquatic, others terrestrial, and some that live on land only as adults. Adult salamanders usually have a generalized tetrapod body plan with four limbs and a tail. Some salamanders are lungless, and respiration occurs through the skin or external gills. Some terrestrial salamanders have primitive lungs; a few species have both gills and lungs.
Figure 6.5 (a) Most salamanders have legs and a tail, but respiration varies among species. (b) The Australian green tree frog is a nocturnal predator that lives in the canopies of trees near a water source. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Frogs (Figure 6.5b) are the most diverse group of amphibians, with approximately 5,000 species that live on all continents except Antarctica. Frogs have a body plan that is more specialized than the salamander body plan for movement on land. Adult frogs use their hind limbs to jump many times their body length on land. Frogs have a number of modifications that allow them to avoid predators, including skin that acts as camouflage and defensive chemicals that are poisonous to predators secreted from glands in the skin.
Frog eggs are fertilized externally, as they are laid in moist environments. Frogs demonstrate a range of parental behaviors, with some species exhibiting little care, to species that carry eggs and tadpoles on their hind legs or backs. The life cycle consists of two stages: the larval stage followed by metamorphosis to an adult stage. The larval stage of a frog, the tadpole, is often a filter-feeding herbivore. Tadpoles usually have gills, a lateral line system, long-finned tails, but no limbs. At the end of the tadpole stage, frogs undergo a gradual metamorphosis into the adult form. During this stage, the gills and lateral line system disappear, and four limbs develop. The jaws become larger and are suited for carnivorous feeding, and the digestive system transforms into the typical short gut of a predator. An eardrum and air-breathing lungs also develop. These changes during metamorphosis allow the larvae to move onto land in the adult stage (Figure 6.6).
Figure 6.6 A frog begins as a (a) tadpole and undergoes metamorphosis to become (b) a juvenile and finally (c) an adult. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Caecilians comprise an estimated 185 species. They lack external limbs and resemble giant earthworms. They inhabit soil and are found primarily in the tropics of South America, Africa, and southern Asia where they are adapted for a soil-burrowing lifestyle and are nearly blind. Unlike most of the other amphibians that breed in or near water, reproduction in a drier soil habitat means that caecilians must utilize internal fertilization, and most species give birth to live young (Figure 6.7)
Figure 6.7 Caecilians lack external limbs and are well adapted for a soil-burrowing lifestyle. Source: https://openstax.org/books/conceptsbiology/pages/15-6-vertebrates
6.3.2.3 Ecological Adaptation of Amphibians
The amphibians are as class, typically furnished with five fingered limbs which are adapted for locomotion both on land as well as in water. They are cold –blooded or poikilothermic animals of jumping and swimming habits. Their skin is smooth, clammy and naked, that is without scales. There is remarkable difference between the young and the adults; the limbless but tailed young forms or tadpoles, as they are called, live in water and breathe by gills while almost in every case the adults breathe by lungs and can live both on land as well as in water and hence the name amphibian. The skin is usually soft and moist in order to carry on the important functions of skin or cutaneous respiration and this is the reason why the presence of moisture is essential for the wellbeing of all amphibians. The tailless amphibians, such as frogs and toads, numbering some thousand species, have a worldwide distribution. They are most abundantly found in tropical countries. Bu it is remarkable that notwithstanding their natural inclination to live in and around water no amphibian has been discovered to live in salt and marine waters. Is a crocodile an amphibian?
Self-Assessment Exercises 3
1. Which animals are called amphibians?
2. What is considered an amphibian?
6.4 Summary
You have studied in this unit that fishes are adapted to life in water, with specialized structures like gills for breathing and fins for swimming, while amphibians have adaptations that allow them to live both in water and on land, including their unique skin and specialized limbs
The earliest vertebrates that diverged from the invertebrate chordates were the jawless fishes. Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and some species are parasitic on other fishes. Gnathostomes include the jawed fishes (cartilaginous and bony fishes) as well as all other tetrapods. Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Bony fishes can be further divided into ray-finned and lobe-finned fishes.
As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of salamanders and all caecilians are limbless. Amphibians have a moist, permeable skin used for cutaneous respiration. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of amphibians consists of two distinct stages: the larval stage and metamorphosis to an adult stage.
UNIT 7 VERTEBRATES II: REPTILES, BIRDS AND MAMMALS
Unit structure
7.1 Introduction
7.2 Intended Learning Outcomes (ILOs)
7.3 Main Contents
7.3.1 Vertebrates II: The Amniotes
7.3.1.1 Reptiles
7.3.1.2 Diversity of Reptiles
7.3.2 Birds
7.3.2.1 Characteristics of Birds
7.3.2.2 External Features of Birds
7.3.2.3 Adaptive Features of Birds to Their Environment
7.3.3 Mammals
7.3.3.1 Diversity and Characteristics of Mammals
7.3.3.2 Primates
7.3.3.3 Adaptations of vertebrates to the terrestrial environment
7.4 Summary
7.1 Introduction
In this unit, we shall study about the amniotes which are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes are characterized by having an egg equipped with an amnion, an adaptation to lay eggs on land or retain the fertilized egg within the mother. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians.
7.2 Intended Learning Objectives (ILOs)
1. Identify the classes of animals that are amniotes
2. Explain the main characteristics of amphibians, reptiles, and birds
3. Discuss the evolution of amniotes
4. Describe the derived characteristics in birds that facilitate flight
5. Name and describe the distinguishing features of the three main groups of mammals
6. Describe the derived features that distinguish primates from other animal
7.3.1 Vertebrates II:
The Amniotes The amniotes—reptiles, birds, and mammals—are distinguished from amphibians by their terrestrially adapted (shelled) egg and an embryo protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes could develop within an aquatic environment inside the egg. This led to less dependence on a water environment for development and allowed the amniotes to invade drier areas. This was a significant evolutionary change that distinguished them from amphibians, which were restricted to moist environments due to their shell-less eggs. Although the shells of various amniotic species vary significantly, they all allow retention of water. The membranes of the amniotic egg also allowed gas exchange and sequestering of wastes within the enclosure of an eggshell. The shells of bird eggs are composed of calcium carbonate and are hard and brittle, but possess pores for gas and water exchange. The shells of reptile eggs are more leathery and pliable. Most mammals do not lay eggs; however, even with internal gestation, amniotic membranes are still present
In the past, the most common division of amniotes has been into classes Mammalia, Reptilia, and Aves. Birds are descended, however, from dinosaurs, so this classical scheme results in groups that are not true clades. We will discuss birds as a group distinct from reptiles with the understanding that this does not reflect evolutionary history.
7.3.1.1Reptiles
They are basically Tetrapods. Snakes and other squamates have vestigial organs such as limbs and are categorised as tetrapods, like caecilians, since they are descended from four-limbed ancestors. Reptiles deposit eggs encased in shells on land. Even reptiles that live in the water come ashore to lay their eggs. Internal fertilisation is the most common method of reproduction for them. Some species are ovoviviparous, meaning the eggs stay in the mother's body until the time comes for them to hatch. Other species are viviparous, meaning they give birth to live offspring. Which vertebrate class is considered to be the first that was entirely terrestrial?
1. Reptiles have Oviparous Characteristics: The majority of reptiles reproduce sexually, however others can reproduce asexually. The cloaca, which is located at the base of the tail, is used for reproduction. Most reptiles have copulatory organs, which are generally hidden inside their bodies. A penis is seen in male turtles and crocodiles, whereas a pair of hemipenes is found in lizards and snakes. Because some species, such as the tuatara, lack copulatory organs, mating is accomplished by forcing the cloaca together.
Figure 7.1 Life cycle of a Turtle. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Life Cycle of Reptiles
1. Reptiles have scales: The creation of reptiles' scaly skin, which includes the protein keratin and waxy lipids to reduce water loss from the skin, was one of the important adaptations that allowed them to survive on land. Because reptiles and amphibians have occlusive skin, they can't breathe through their skin and must rely on their lungs.
2. Reptiles are Ectotherms: Ectotherms are animals whose primary source of body heat is derived from their surroundings.
Endotherms, on the other hand, control body temperature using heat produced by metabolism. Reptiles are classified as poikilotherms, or creatures whose body temperatures fluctuate rather than remain constant, in addition to being ectothermic. Reptiles have behavioural adaptations to assist control body temperature, such as warming up by basking in the sun and cooling down by choosing shaded regions or burrowing underground.
3. Lungs are used by reptiles to breathe: Lungs are used by reptiles to breathe. The breathing process can only be completed through the lungs, despite the fact that turtles have porous skin through which gaseous exchange occurs, and certain species improve the pace of gaseous exchange through the cloaca. Distinct reptile species have different ways of breathing through their lungs. The lungs of lizards and snakes are ventilated by axial musculature, which is also utilised to move. As a result of these factors, the majority of animals are obliged to hold their breath during strenuous activity. Some lizards, on the other hand, use buccal pumping to help them breathe. Crocodiles have muscular diaphragms, which pull the pubis backwards to provide room for the lungs to expand. Because certain reptiles lack secondary palates, they must hold their breath while swallowing.
Figure 7.2 Breathing lungs in reptiles. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Respiratory System of Reptiles
1. Reptiles have characteristics of a Vertebrate: Reptiles have traits that are comparable to those of mammals, birds, and certain amphibians. The spinal cords that span the length of their bodies are housed in their backbones. From the tail to the skull, reptiles contain a chain of bone parts. The cranium or skull, appendages, and limb girdles make up the bone endoskeleton. The inner tissue is protected by the endoskeleton, which also assists in bodily mobility. What constitutes the bone exoskeleton in reptiles?
Self-Assessment Exercises 1
1. How does reproduction atake place in animals of the class Reptilia?
2. Do organisms of the class Reptilia have direct or indirect development?
7.3.1.2 Diversity of Reptiles
Class Reptilia includes diverse species classified into four living clades. These are the Crocodilia, Sphenodontia, Squamata, and Testudines. The Crocodilia (“small lizard”) arose approximately 84 million years ago, and living species include alligators, crocodiles, and caimans. Crocodilians (Figure 7.3a) live throughout the tropics of Africa, South America, the southeastern United States, Asia, and Australia. They are found in freshwater habitats, such as rivers and lakes, and spend most of their time in water. Some species are able to move on land due to their semi-erect posture.
Figure 7.3 (a) Crocodilians, such as this Siamese crocodile, provide parental care for their offspring. (b) This Jackson’s chameleon blends in with its surroundings. (c) The garter snake belongs to the genus Thamnophis, the most widely distributed reptile genus in North America. (d) The African spurred tortoise lives at the southern edge of the Sahara Desert. It is the third largest tortoise in the world. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
The Sphenodontia (“wedge tooth”) arose in the Mesozoic Era and includes only one living genus, Tuatara, with two species that are found in New Zealand. There are many fossil species extending back to the Triassic period (250–200 million years ago). Although the tuataras resemble lizards, they are anatomically distinct and share characteristics that are found in birds and turtles.
Squamata (“scaly”) arose in the late Permian; living species include lizards and snakes, which are the largest extant clade of reptiles (Figure 7.3b). Lizards differ from snakes by having four limbs, eyelids, and external ears, which are lacking in snakes. Lizard species range in size from chameleons and geckos that are a few centimeters in length to the Komodo dragon, which is about 3 meters in length.
Snakes are thought to have descended from either burrowing lizards or aquatic lizards over 100 million years ago (Figure 7.3c). Snakes comprise about 3,000 species and are found on every continent except
Antarctica. They range in size from 10 centimeter-long thread snakes to 7.5 meter-long pythons and anacondas. All snakes are carnivorous and eat small animals, birds, eggs, fish, and insects.
Turtles are members of the clade Testudines (“having a shell”) (Figure 7.3d). Turtles are characterized by a bony or cartilaginous shell, made up of the carapace on the back and the plastron on the ventral surface, which develops from the ribs. Turtles arose approximately 200 million years ago, predating crocodiles, lizards, and snakes. Turtles lay eggs on land, although many species live in or near water. Turtles range in size from the speckled padloper tortoise at 8 centimeters (3.1 inches) to the leatherback sea turtle at 200 centimeters (over 6 feet). The term “turtle” is sometimes used to describe only those species of Testudines that live in the sea, with the terms “tortoise” and “terrapin” used to refer to species that live on land and in fresh water, respectively. Compared to amphibians, what is an example of an evolutionary innovation present in organisms of the class Reptilia to combat the loss of water through the skin? Reptile skin is keratinized and impermeable to water whereas amphibian skin is permeable. The impermeability of their skin made the cutaneous gas exchange performed by amphibians impossible, making respiration dependent on internal organs such as airways and lungs. Reptiles have developed several adaptations to lead a complete terrestrial life.
These adaptations include:
1. Thick, scaly skin that helps them to conserve moisture inside
their bodies, which helps them survive on land (especially in hot and
dry areas).
2. Efficient excretory systems which help them to excrete a highly
concentrated urine.
3. Kidneys that have adapted to living on land with limited access to
drinking water.
4. A reproductive strategy that involves laying soft-shelled eggs.
5. Lungs that are adapted to breathing air.
6. Basking in the sun to regulate their body temperature
7. Legs that are adapted for walking on land
7.3.2 Birds
The most obvious characteristic that sets birds apart from other modern vertebrates is the presence of feathers, which are modified scales. While vertebrates like bats fly without feathers, birds rely on feathers and wings, along with other modifications of body structure and physiology, for flight.
7.3.2.1 Characteristics of Birds
Birds are endothermic, and because they fly, they require large amounts of energy, necessitating a high metabolic rate. Like mammals, which are also endothermic, birds have an insulating covering that keeps heat in the body: feathers. Specialized feathers called down feathers are especially insulating, trapping air in spaces between each feather to decrease the rate of heat loss. Certain parts of a bird’s body are covered in down feathers, and the base of other feathers have a downy portion, whereas newly hatched birds are covered in down.
Feathers not only act as insulation but also allow for flight, enabling the lift and thrust necessary to become airborne. The feathers on a wing are flexible, so the collective feathers move and separate as air moves through them, reducing the drag on the wing. Flight feathers are asymmetrical, which affects airflow over them and provides some of the lifting and thrusting force required for flight (see the figure below).
Two types of flight feathers are found on the wings, primary feathers and secondary feathers. Primary feathers are located at the tip of the wing and provide thrust. Secondary feathers are located closer to the body, attach to the forearm portion of the wing and provide lift. Contour feathers are the feathers found on the body, and they help reduce drag produced by wind resistance during flight. They create a smooth, aerodynamic surface so that air moves smoothly over the bird’s body, allowing for efficient flight.
Figure 7.4 (a) Primary feathers are located at the wing tip and provide thrust; secondary feathers are located close to the body and provide lift. (b) Many birds have hollow pneumatic bones, which make flight easier. Source:https://openstax.org/books/concepts-biology/pages/15-6- vertebrates
Flapping of the entire wing occurs primarily through the actions of the chest muscles, the pectoralis and the supracoracoideus. These muscles are highly developed in birds and account for a higher percentage of body mass than in most mammals. These attach to a blade-shaped keel, like that of a boat, located on the sternum. The sternum of birds is larger than that of other vertebrates, which accommodates the large muscles required to generate enough upward force to generate lift with the flapping of the wings. Another skeletal modification found in most birds is the fusion of the two clavicles (collarbones), forming the furcula or wishbone. The furcula is flexible enough to bend and provide support to the shoulder girdle during flapping.
An important requirement of flight is a low body weight. As body weight increases, the muscle output required for flying increases. The largest living bird is the ostrich, and while it is much smaller than the largest mammals, it is flightless. For birds that do fly, reduction in body weight makes flight easier. Several modifications are found in birds to reduce body weight, including pneumatization of bones. Pneumatic bones are bones that are hollow, rather than filled with tissue (see the figure below). They contain air spaces that are sometimes connected to air sacs, and they have struts of bone to provide structural reinforcement. Pneumatic bones are not found in all birds, and they are more extensive in large birds than in small birds. Not all bones of the skeleton are pneumatic, although the skulls of almost all birds are.
Figure 7.5 Birds have hollow, pneumatic bones, which make flight easier. Source: https://openstax.org/books/concepts-biology/pages/15-6- vertebrates
Other modifications that reduce weight include the lack of a urinary bladder. Birds possess a cloaca, a structure that allows water to be reabsorbed from waste back into the bloodstream. Uric acid is not expelled as a liquid but is concentrated into urate salts, which are expelled along with fecal matter. In this way, water is not held in the urinary bladder, which would increase body weight. Most bird species only possess one ovary rather than two, further reducing body mass. The air sacs that extend into bones to form pneumatic bones also join with the lungs and function in respiration. Unlike mammalian lungs in which air flows in two directions, as it is breathed in and out, airflow through bird lungs travels in one direction (see the figure below). Air sacs allow for this unidirectional airflow, which also creates a cross-current exchange system with the blood. In a cross-current or counter-current system, the air flows in one direction and the blood flows in the opposite direction, creating a very efficient means of gas exchange.
What is the role of the cloaca in the success of birds for live in the terrestrial environment?
Figure 7.6 Avian respiration is an efficient system of gas exchange with air flowing unidirectionally. During inhalation, air passes from the trachea into posterior air sacs, then through the lungs to anterior air sacs. The air sacs are connected to the hollow interior of bones. During exhalation, air from air sacs passes into the lungs and out the trachea. Source: https://openstax.org/books/concepts-biology/pages/15-6- vertebrates
7.3.2.2 External Features of Birds
The body of the birds is steamlined or spindle-shaped well adapted for aerial life. The neck is usually long and flexible. The head is rounded and the facial portion is produced into beak. Close to the base of the beak are two slit-like nostrils. The eyes of the bird are of considerable size for the sense of smell seems to be feebly developed but the powervision is correspondingly advanced, especially in some birds of prey. Each eye is provided with three eyelids: the upper and lower eyelids are like fold of skin while the third eyelid, the nictitating membrane, is a delicate transparent membrane which can be drawn across the eye. Usually the lower lid is movable. On each side of the head there is a small aperture with a short passage leading to the eardrum. It lies behind and below the eyes and is usually hidden by the features. Unlike mammals, the external ears are absent. What are the main morphological features of birds?
7.3.2.3 Adaptive Features of Birds to Their Environment
The fore limbs form the wings which are organs of flight while the hind limbs are adapted for bearing the entire weight of the body when walking. For this purpose the hind limbs are usually attached somewhat for forward and the skeleton is also modified to this end. The legs are covered with scales. The cloaca lies on the ventral surface at the root of the tail and on the dorsal surface of the same region is an oilgland. Its oily secretion is used for preening feathers. The feet, break and the tongue present very large number of variations of form which are closely associated with the habits of the birds. The typical number of the toes is four, of which, three are directed forwards and one backwards. In perching birds the toes are adapted for grasping and automatically clutching the support. Three toes are directed forwards and one backwards. The same arrangement of the toes is also found in birds of prey which use their feet for seizing. The claws form great talons as in the eagles, hawks, kites, falcons, etc. The legs of the wading birds are usually very long and partly or completely unfeathered up to the tibial region. They have very long toes. Swimming birds like ducks have webbed feet which serve as paddles. What adaptations for flight are present in birds?
Self-Assessment Exercises 2
1. What adaptations for flight are present in birds?
2. What similarities are present in birds and reptiles regarding external coverage, reproduction and excretion?
Mammals
Mammals are vertebrates that have hair and mammary glands used to provide nutrition for their young. Certain features of the jaw, skeleton, skin, and internal anatomy are also unique to mammals. The presence of hair is one of the key characteristics of a mammal. Although it is not very extensive in some groups, such as whales, hair has many important functions for mammals. Mammals are endothermic, and hair provides insulation by trapping a layer of air close to the body to retain metabolic heat. Hair also serves as a sensory mechanism through specialized hairs called vibrissae, better known as whiskers. These attach to nerves that transmit touch information, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration.
Mammalian skin includes secretory glands with various functions. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin for water resistance and lubrication. Sebaceous glands are located over most of the body. Sudoriferous glands produce sweat and scent, which function in thermoregulation and communication, respectively. Mammary glands produce milk that is used to feed newborns. While male and female monotremes and eutherians possess mammary glands, some male marsupials do not. The skeletal system of mammals possesses unique features that differentiate them from other vertebrates. Most mammals have heterodont teeth, meaning they have different types and shapes of teeth that allow them to feed on different kinds of foods. These different types of teeth include the incisors, the canines, premolars, and molars. The first two types are for cutting and tearing, whereas the latter two types are for crushing and grinding. Different groups have different proportions of each type, depending on their diet. Most mammals are also diphyodonts, meaning they have two sets of teeth in their lifetime: deciduous or “baby” teeth, and permanent teeth. In other vertebrates, the teeth can be replaced throughout life
Modern mammals are divided into three broad groups: monotremes, marsupials, and eutherians (or placental mammals). The eutherians, or placental mammals, and the marsupials collectively are called therian mammals, whereas monotremes are called prototherians.
7.3.3.1Diversity and Characteristics of Mammals
There are three living species of monotremes: the platypus and two species of echidnas, or spiny anteaters (Figure 7.7). The platypus and one species of echidna are found in Australia, whereas the other species of echidna is found in New Guinea. Monotremes are unique among mammals, as they lay leathery eggs, similar to those of reptiles, rather than giving birth to live young. However, the eggs are retained within the mother’s reproductive tract until they are almost ready to hatch. Once the young hatch, the female begins to secrete milk from pores in a ridge of mammary tissue along the ventral side of her body. Like other mammals, monotremes are endothermic but regulate body temperatures somewhat lower (90 °F, 32 °C) than placental mammals do (98 °F, 37 °C). Like reptiles, monotremes have one posterior opening for urinary, fecal, and reproductive products, rather than three separate openings like placental mammals do. Adult monotremes lack teeth.
Figure 7.7 The platypus (left), a monotreme, possesses a leathery beak and lays eggs rather than giving birth to live young. An echidna, another monotreme, is shown in the right photo. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
Marsupials are found primarily in Australia and nearby islands, although about 100 species of opossums and a few species of two other families are found in the Americas. Australian marsupials number over 230 species and include the kangaroo, koala, bandicoot, and Tasmanian devil (Figure 7.8). Most species of marsupials possess a pouch in which the young reside after birth, receiving milk and continuing to develop. Before birth, marsupials have a less complex placental connection, and the young are born much less developed than in placental mammals. What are the three main groups into which mammals are divided?
Figure 7.8 The Tasmanian devil is one of several marsupials native to Australia.Source: https://openstax.org/books/concepts-biology/pages/15- 6-vertebrates
Eutherians are the most widespread of the mammals, occurring throughout the world. There are several groups of eutherians, including Insectivora, the insect eaters; Edentata, the toothless anteaters; Rodentia, the rodents; Chiroptera, the bats; Cetacea, the aquatic mammals including whales; Carnivora, carnivorous mammals including dogs, cats, and bears; and Primates, which includes humans.
Eutherian mammals are sometimes called placental mammals, because all species have a complex placenta that connects a fetus to the mother, allowing for gas, fluid, waste, and nutrient exchange. While other mammals may possess a less complex placenta or briefly have a placenta, all eutherians have a complex placenta during gestation.
The main Characteristics of a mammal are as follows: It is warmblooded. Its skin has sweat and sebaceous glands and a covering of hairs. It has different types of teeth, with each type carrying out a specific function. It has external ears called pinnae. Its body cavity is separated into two by a muscular sheet called a diaphragm. The upper thoracic cavity contains the lungs and the heart while the lower abdominal cavity contains the alimentary canal, the Kidneys and the reproductive organs. It has a well-developed heart. It has a welldeveloped brain. Fertilization is internal. In most mammals, the tiny fertilized egg develops inside the body of the female parent for a period. During this time, the young is attached to the mother by a placenta, an organ through which it obtains nourishment from the mother. The young is born alive (Vivipary) and feeds on the milk secreted by the mother’s mammary glands. It is looked after by the parents until it learns to be independent.
7.3.3.2Primates
Order Primates of class Mammalia includes lemurs, tarsiers, monkeys, and the apes, which include humans. Non-human primates live primarily in tropical or subtropical regions of South America, Africa, and Asia. They range in size from the mouse lemur at 30 grams (1 ounce) to the mountain gorilla at 200 kilograms (441 pounds). The characteristics and evolution of primates are of particular interest to us as they allow us to understand the evolution of our own species.
All primate species have adaptations for climbing trees, as they all descended from tree-dwellers, although not all species are arboreal. This arboreal heritage of primates resulted in hands and feet that are adapted for brachiation, or climbing and swinging through trees. These adaptations include, but are not limited to 1) a rotating shoulder joint, 2) a big toe that is widely separated from the other toes and thumbs that are widely separated from fingers (except humans), which allow for gripping branches, and 3) stereoscopic vision, two overlapping visual fields, which allows for the depth perception necessary to gauge distance. Other characteristics of primates are brains that are larger than those of many other mammals, claws that have been modified into flattened nails, typically only one offspring per pregnancy, and a trend toward holding the body upright.
Order Primates is divided into two groups: prosimians and anthropoids. Prosimians include the bush babies of Africa, the lemurs of Madagascar, and the lorises, pottos, and tarsiers of Southeast Asia. Anthropoids include monkeys, lesser apes, and great apes (Figure 7.9). In general, prosimians tend to be nocturnal, smaller in size than anthropoids, and have relatively smaller brains compared to anthropoids.
Figure 7.9 Primates can be divided into prosimians, such as the (a) lemur, and anthropoids. Anthropoids include monkeys, such as the (b) howler monkey; lesser apes, such as the (c) gibbon; and great apes, such as the (d) chimpanzee, (e) bonobo, (f) gorilla, and (g) orangutan. Source: https://openstax.org/books/concepts-biology/pages/15-6-vertebrates
7.3.3.3Adaptations of Vertebrates to the Terrestrial Environment
The animals, when they left the aquatic environment to colonize the terrestrial environment , had to face the problem of desiccation. Only those that had the necessary adaptations to avoid desiccation were able to conquer the terrestrial environment. Some of these adaptations are:
1. The skin: A skin is needed to isolate the organism from the environment and prevent water loss. Reptiles have their skin covered with scales, which prevents it from growing. This is the reason they have to shed their skin. Mammals have fur covered skin and Birds have feathers cover their skin.
1. Efficient kidneys allow more water to be used by reducing its loss in excretion.
2. Movement: terrestrial vertebrates have four legs with which they can move or manipulate.
3. Lungs: necessary to capture oxygen from the air and be able to carry it, through the circulatory system, to the cells. Amphibians have their lungs are rudimentary, they need cutaneous respiration as a complement, and Birds and mammals have their lungs are more evolved
Reproduction: Amphibians is the group worst adapted to life on land. Their fertilization is external, they lay eggs in the water and the young cannot leave the water until they become adults. The rest of tetrapods have sexual reproduction with internal fertilization. While Reptiles and birds are oviparous. The bone has a hard shell and a protective bag, the amnion, filled with a liquid that protects and isolates the embryo. Normally, the mother deposits the eggs and the embryo does not come out until it is already developed. In some cases, they are ovoviviparous, and the eggs remain inside the mother until the young hatch. Mammals, except for the platypus, are viviparous. The embryo is not protected by an egg, but instead develops in the mother's womb and is nourished through the umbilical cord (except in marsupials). When they are born, they feed on mother's milk. How do placental mammals reproduce?
Vertebrates are adapted to life underground, on the surface, and in the air. They range in size from minute fishes to elephants and whales. Vertebrates feed upon plants, invertebrate animals, and one another. Vertebrate faunas are important to humans for food and recreation. Important adaptations of vertebrates include: 1. Structural adaptations, such as the color of skin, body shape, and body covering. 2. Behavioral adaptations, such as special behavioral changes that allow an organism to survive in its natural habitat.
Self-Assessment Exercises 3
1. What are the typical features of mammals?
2. How is circulation characterized in mammals?
7.4 Summary
The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. A key adaptation that permitted reptiles to live on land was the development of scaly skin. Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).
Birds are endothermic amniotes. Feathers act as insulation and allow for flight. Birds have pneumatic bones that are hollow rather than tissuefilled. Airflow through bird lungs travels in one direction. Birds evolved from dinosaurs
Mammals have hair and mammary glands. Mammalian skin includes various secretory glands. Mammals are endothermic, like birds. There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are unique among mammals as they lay eggs, rather than giving birth to live young. Eutherian mammals have a complex placenta.
Glossary
Abdomen: Region of the body furthest from the mouth. In insects, the third body region behind the head and thorax.
Altricial: Refers to animals with young that are unable to move on their own after hatching or birth, and require extensive parental care.
Ambulacra: Row of tube feet of an echinoderm.amniotic egg:An egg that can be laid on land due to the presence of a fluid-filled amniotic sac (amnion) that cushions and protects the developing embryo.
Amniote: Any of a group of land-dwelling vertebrates that have an amnion during embryonic development, including reptiles, birds, and mammals.
Anapsid: A vertebrate distinguished by a skull with no openings in the side behind the eyes, e.g. turtles.
Anus: End of the digestive tract, or gut, through which waste products of digestion are excreted, as distinct from the mouth.
Bipedal: Describes an animal that walks on two legs.
Biramous: Arthropod appendages that are biramous have two branches, an outer branch and an inner branch.
Blood: Fluid which circulates throughout the body of an animal, distributing nutrients, and often oxygen as well.
book lung: A set of soft overlapping flaps, covered up by a plate on the abdomen, through which oxygen is taken up and carbon dioxide given off.
Brain: Collection of nerve cells usually located at the anterior end of an animal, when present at all.
Cephalon: In trilobites, the head shield bearing the eyes, antennae, and mouth.
Chaetae: Stiff bristles characteristic of annelids.
Chela: The claw of an arthropod.
Chelicera: The first pair of appendages of a chelicerate arthropod. Originally a short clawed appendage, the chelicerae of many arachnids are highly modified for feeding.
Chordate: an animal with a notochord (a cartilaginous rod that extends the length of the body), dorsal hollow nerve cord (a fluid-filled tube that runs the length of the body), gill slits or pouches, and a tail at some stage in its life cycle.
Clitellum: In annelids, a swelling of the body towards the head of the animal, where the gonads are located.
Cnidocyst: The "stinging cell" of a cnidarian.
Coelom: Fluid-filled cavity within the body of an animal; usually refers to a cavity lined with specialized tissue peritoneum in which the gut is suspended.
compound eye: found in many but not all arthropods, a compound eye is composed of a large number of small, closely packed simple eyes (ommatidia), each with its own lens and nerve receptors.
Cuticle: In animals, a multilayered, extracellular, external body covering, usually composed of fibrous molecules such as chitin or collagen, and sometimes strengthened by the deposition of minerals such as calcium carbonate.
Diapsid: A vertebrate distinguished by a skull with two pairs of openings in the side behind the eyes, e.g., lizards, snakes, crocodiles, dinosaurs, and pterosaurs.
Ectoderm: The outer basic layer of tissue in those animals with true tissues.
Endoderm: The innermost basic layer of tissue in those animals with true tissues.
Epidermis: The outermost layer of cells or skin. This tissue often contains specialized cells for defense, gas exchange, or secretion.
Epithelium: Layer of cells which lines a body cavity; cells may be ciliated or unciliated, and may be squamous (flat, scale shaped), cuboidal (cube-shaped), or columnar (columnshaped).
Esophagus: That portion of the gut which connects the pharynx to the stomach.
Exoskeleton: An external, often hard, covering or integument that provides support and protection to the body.
Gastrodermis: In cnidarians, the endodermis which lines the gut cavity.
Genus: A category in the classification of plants and animals between species and family; genera- pl.
Gill: In aquatic animals, highly vascularized tissues with large surface area; these are extended out of the body and into the surrounding water for gas exchange.
gill arches: Stiffenings which support the flesh between the gill slits of chordates.
gill slit: A slitlike or porelike opening connecting the pharynx of a chordate with the outside of the body.
Gnathobase: The expanded and hardened base of the appendage of many arthropods, notably trilobites, crustaceans, and marine cheliceramorphs.
gut (enteron): Body cavity formed between the mouth and anus in which food is digested and nutrients absorbed; it consists of the mouth, pharynx, esophagus, stomach, intestine,and anus, though some animals do not have all these regions.
Head: That part of the body at the "front" end, where the brain, mouth, and most sensory organs are located.
Heart: Muscular pump which circulates the blood.
Histostructure: The organization and arrangement of tissue
Incubation: In birds and reptiles, the maintaining of a constant temperature during the development of the embryo.
Intestine: The portion of the digestive tract between the stomach and anus; it is the region where most of the nutrients and absorbed.
Jaw: Often loosely applied to any movable, toothed structures at or near the mouth of an animal, such as the scolecodonts of annelids.
Jointed: When stiff body parts are connected by a soft flexible region, the body is said to be jointed.
Librigenae: The "free cheeks"; separate, detachable portions of the trilobite cephalon.
Lophophore: Complex ring of hollow tentacles used as a feeding organ.
Mammilla: In eggshell, the cone-like structure at the base of the shell unit where the shell unit attaches to the inner organic membrane.
Marsupial: A mammal that gives live birth to young that have gestated for only a short period of time.
Mesoderm: In animals with three tissue layers (i.e. all except sponges and cnidarians), the middle layer of tissue, between the ectoderm and the endoderm.
Mesogloea: Jellylike material between the outer ectoderm and the inner endoderm of cnidarians.
Metabolism: The chemical processes within an organic body that supply the energy necessary for life.
Microstructure: In eggshell, the shape, size, orientation, and distribution of components of the shell.
Monotreme: A mammal that lays eggs rather than giving live birth.
Mouth: Front opening of the digestive tract, into which food is taken for digestion.
Mucus: Sticky secretion used variously for locomotion, lubrication, or protection from foreign particles.
Muscle: Bundle of contractile cells which allow animals to move.
Myotome: Segment of the body formed by a region of muscle.
Nematocyst: Older name for a cnidocyst.
Nerve: A bundle of neurons, or nerve cells.
Nerve cord: Primary bundle of nerves in chordates, which connects the brain to the major muscles and organs of the body.
Neuron: A specialized cell that can react to stimuli and transmit impulses.
Notochord: Characteristic of chordates, the notochord is a stiff rod of tissue along the back of the body.
Organ: Collection of tissues which performs a particular function or set of functions in an animal or plant's body.
organ system: Collection of organs which have related roles in an organism's functioning.
Osculum: The main opening through which filtered water is discharged.
Ovulation: The process by which an egg (the female gamete) is released from the ovary.
papilla(e): Cellular outgrowths.
Parapodia: A sort of "false foot" formed by extension of the body cavity.
Pathology: The study of disease and abnormalities.
Pedipalps: The second pair of appendages of cheliceromorphs.
Pharyngeal slits: Characteristic of chordates, pharyngeal slits are openings through which water is taken into the pharynx, or throat.
Pharynx: Cavity in the digestive tract just past the mouth itself.
Phylum: A category in the hierarchy of animal classification between class and kingdom; phyla- pl.
Placenta: In mammals, a tissue formed within the uterus through which nutrients are passed from the mother to the embryo (and later the fetus) and its wastes are removed.
Pleurae: In trilobites and other arthropods, pleurae are elongated flat outgrowths from each body segment, that overlie and protect the appendages.
Pore: Any opening into or through a tissue or body structure.
Precocial: Describes young that are mobile and fairly self sufficient at birth.
Proboscis: Elongated organ, usually associated with the mouth.
Pygidium: In trilobites, the posterior division of the body, formed by fusion of the telson with one or more posterior pleurae.
Segmentation: In many animals, the body is divided into repeated subunits called segments, such as those in centipedes, insects, and annelids.
Septum: Partition which divides up a larger region into smaller ones, such as in the central body cavity of some anthozoa.
Siphon: Opening in molluscs or in urochordates which draws water into the body cavity.
Skeleton: Support structure in animals, against which the force of muscles acts.
Spicule: Crystalline or mineral deposits found in sponges, sea cucumbers, or urochordates.
Spiracle: In insects and some other terrestrial arthropods, a small opening through which air is taken into the tracheae.
Spongocoel: Central body cavity of sponges.
Synapsid: A vertebrate distinguished by a skull with one pair of openings in the sidebehind the eyes, e.g., mammals and their close relatives.
Telson: The last segment of the abdomen in many arthropods.
Tentacles: Appendages which are flexible, because they have no rigid skeleton.
Tetrapod: An animal with four limbs that evolved from a common fish ancestor during the Devonian Period (~365 million years ago).
Thorax: In insects, the second body region, between the head and thorax. It is the region where the legs and wings are attached.
Tissue: A group of cells with a specific function in the body of an organism.
Tracheae: Internal tubes through which air is taken for respiration.
Tube feet: Extensions of the water-vascular system of echinoderms, protruding from the bodyand often ending in suckers.
Tubercle: Any small rounded protrusion.
Uniramious: Among arthropods, uniramous refers to appendages that have only one branch.
Vascular: Refers to a network of tubes which distribute nutrients and remove wastes from the tissues of the body.
Vertebra: A component of the vertebral column, or backbone, found in vertebrates.
Zooxanthellae: Symbiotic dinoflagellates in the genus Symbiodinium that live in the tissues of a number of marine invertebrates and protists, notably in many foraminiferans, cnidarians, and some mollusks.
- Lecturer : Nnamdi Chukwu Amadi