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Chapter
Overview of Animal
Diversity
Chapter Outline
32.1 Some General Features of Animals 32.2 Evolution of the Animal Body Plan 32.3 The Classification of Animals 32.4 The Roots of the Animal Tree of Life
Introduction
We now explore the great diversity of modern animals, the result of a long evolutionary history. Animals are among the most abundant living organisms. Found in almost every habitat, they bewilder us with their diversity in form, habitat, behavior, and lifestyle. About a million and a half species have been described, and several million more are thought to await discovery. Despite their great diversity, animals have much in common. For example, locomotion is a distinctive characteristic, although not all animals can move about. Early naturalists thought that sponges and corals were plants because the adults are attached to the surface on which they live.
CHAPTER
T A B L E 3 2. 1 General Features of Animals
Heterotrophy. All animals are heterotrophs—that is, they obtain energy and organic molecules by ingesting other organisms. Unlike autotrophic plants and algae, animals cannot construct organic molecules from inorganic chemicals. Some animals (herbivores) consume autotrophs; other animals (carnivores) consume heterotrophs; some animals (omnivores) consume both autotrophs and heterotrophs; and still others (detritivores) consume decomposing organisms.
Multicellularity. All animals are multicellular; many have complex bodies like that of this jellyfish (phylum Cnidaria). The unicellular heterotrophic organisms called Protozoa, which were at one time regarded as simple animals, are now considered members of the large and diverse kingdom Protista, discussed in chapter 29.
No Cell Walls. Animal cells differ from those of other multicellular organisms: they lack rigid cell walls and are usually quite flexible. The many cells of animal bodies are held together by extracellular frames of structural proteins such as collagen. Other proteins form unique intercellular junctions between animal cells.
Active Movement. Animals move more rapidly and in more complex ways than members of other kingdoms—this ability is perhaps their most striking characteristic, one directly related to the flexibility of their cells and the evolution of nerve and muscle tissues. A remarkable form of movement unique to animals is flying, a capability that is well developed among vertebrates and insects such as this butterfly (phylum Arthropoda). Many animals cannot move from place to place (they are sessile) or do so rarely or slowly (they are sedentary) although they have muscles or muscle fibers that allow parts of their bodies to move. Sponges, however, have little capacity for movement.
2.2 μ m
tions. Taken together, the universal characteristics and other features of major importance that have exceptions are convinc- ing evidence that animals are monophyletic—that they de- scended from a common ancestor. Table 32.1 describes the general features of animals.
Learning Outcome Review 32.
All animals are multicellular and heterotrophic, and their cells lack cell walls. Most animals can move from place to place, can reproduce sexually, and possess unique tissues. Animals can be found in almost all habitats.
■ What evidence is there that animals could not have been the first type of life to have evolved?
32.1 Some General Features
of Animals
Learning Outcome
1. Identify three features that characterize all animals and three that characterize only some types of animals.
Animals are so diverse that few criteria fit them all. But some, such as animals being eaters, or consumers, apply to all. Others, such as their being mobile (they can move about) have excep-
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a.
Radial Symmetry
b.
Bilateral Symmetry
Ventral
Dorsal
Transverse plane
Frontal plane
Sagittal plane
Anterior
Posterior
Figure 32.1 A comparison of radial and bilateral
symmetry. a. Radially symmetrical animals, such as this sea anemone (phylum Cnidaria), can be bisected into equal halves by any longitudinal plane that passes through the central axis. b. Bilaterally symmetrical animals, such as this turtle (phylum Chordata), can only be bisected into equal halves in one plane (the sagittal plane).
image halves, one along the mouth and one perpendicular to it; these animals are actually biradially symmetrical (figure 32.1 b ).
Bilateral symmetry
The bodies of most animals other than sponges and cnidarians exhibit bilateral symmetry, in which the body has right and left halves that are mirror images of each other. Animals with this body plan are collectively termed the Bilateria. The sagittal plane defines these halves. A bilaterally symmetrical body has, in addi- tion to left and right halves, dorsal and ventral portions, which are divided by the frontal plane, and anterior (front) and poste- rior (rear) ends, which are divided by the transverse plane (in an animal that walks on all fours, dorsal is the top side). In echino- derms (sea stars and their relatives), adults are radially symmetri- cal (actually pentaradially symmetrical, because the body has five clear sections), but the larvae are bilaterally symmetrical.
32.2 Evolution of the Animal
Body Plan
Learning Outcomes
**1. Differentiate between a pseudocoelom and a coelom.
- Explain the difference between protostomes and** **deuterostomes.
- Describe the advantages of segmentation.**
The features described in the preceding section evolved over the course of millions of years. We can understand how the his- tory of life has proceeded by examining the types of animal bodies and body plans present in fossils and in existence today. Five key innovations can be noted in animal evolution:
1. The evolution of symmetry 2. The evolution of tissues, allowing specialized structures and functions 3. The evolution of a body cavity 4. The evolution of various patterns of embryonic development 5. The evolution of segmentation, or repeated body units
These innovations are explained in the sections that follow. Some innovations appear to have evolved only once, some twice or more. Scientists use an innovation that evolved once as evi- dence that all the animals possessing it are more closely related to one another than they are to any animal lacking the innova- tion. The animals with the innovation and their ancestor in which the innovation arose are said to constitute a clade—an evolutionarily coherent group (see chapter 23). On the other hand, some innovations evolve more than once in different clades. This is the phenomenon of convergent evolution (see chapter 23). Although not indicative of close evolutionary rela- tionship, convergently evolved innovations may be important to how species have adapted to their environments.
Most animals exhibit radial
or bilateral symmetry
A typical sponge lacks definite symmetry, growing as an irregu- lar mass. Virtually all other animals have a definite shape and symmetry that can be defined along an imaginary axis drawn through the animal’s body. The two main types of symmetry are radial and bilateral.
Radial symmetry
The body of a member of phylum Cnidaria (jellyfish, sea anemo- nes, and corals: the C of Cnidaria is silent; see chapter 34) exhibits radial symmetry. Its parts are arranged in such a way that any longitudinal plane passing through the central axis divides the organism into halves that are approximate mirror images (figure 32.1 a ). A pie, for example, is radially symmetrical. In cni- darians such as corals and sea anemones, the mouth is not circular, but oval, because it opens into a sort of throat that is like a flattened sleeve. Thus there are two planes that divide the body into mirror-
636 part V Diversity of Life on Earth
Flatworm
Ectodermally derived tissue
Mesodermally derived tissue
Ectodermally derived tissue
Ectodermally derived tissue
Mesodermally derived tissue
Mesodermally derived tissue
Endodermally derived tissue
Acoelomate
Annelid
Coelomate
Coelom
Roundworm
Endodermally derived tissue
Endodermally derived tissue
Pseudocoelomate
Pseudocoelom
Figure 32.2 Three body plans for bilaterally
symmetrical animals. Acoelomates, such as flatworms, have no body cavity between the digestive tract (derived from the endoderm) and the musculature layer (derived from the mesoderm). Pseudocoelomates have a body cavity, the pseudocoelom, between tissues derived from the endoderm and those derived from the mesoderm. Coelomates have a body cavity, the coelom, that develops entirely within tissues derived from the mesoderm, and so is lined on both sides by tissue derived from the mesoderm.
A key innovation in the body plan of some bilaterians was a body cavity isolated from the exterior of the animal. This is differ- ent from the digestive cavity, which is open to the exterior at least through the mouth, and in most animals at the opposite end as well, via the anus. The evolution of efficient organ systems within the animal body was not possible until a body cavity evolved for accommodating and supporting organs (such as our heart and lungs), distributing materials, and fostering complex develop- mental interactions. The cavity is filled with fluid: in most ani- mals, the fluid is liquid, but in vertebrates, it is gas—the body cavity of humans filling with liquid is a life-threatening condition. A very few types of bilaterians have no body cavity, the space be- tween tissues that develop from the mesoderm and those that de- velop from the endoderm being filled with cells and connective tissue. These are the so-called acoelomate animals (figure 32.2).
Bilateral symmetry constitutes a major evolutionary advance in the animal body plan. Bilaterally symmetrical animals have the ability to move through the environment in a consistent direction (typically with the anterior end leading)—a feat that is difficult for radially symmetrical animals. Associated with directional move- ment is the grouping of nerve cells into a brain, and sensory struc- tures, such as eyes and ears, at the anterior end of the body. This concentration of nervous tissue at the anterior end, which appears to have occurred early in evolution, is called cephalization. Much of the layout of the nervous system in bilaterally symmetrical ani- mals is centered on one or more major longitudinal nerve cords that transmit information from the anterior sense organs and brain to the rest of the body. Cephalization is often considered a consequence of the development of bilateral symmetry.
The evolution of tissues allowed for
specialized structures and functions
The zygote (a fertilized egg), has the capability to give rise to all the kinds of cells in an animal’s body. That is, it is totipotent (all powerful). During embryonic development, cells specialize to carry out particular functions. In all animals except sponges, the process is irreversible: once a cell differentiates to serve a func- tion, it and its descendants can never serve any other. A sponge cell that had specialized to serve one function (such as lining the cavity where feeding occurs) can lose the special at- tributes that serve that function and change to serve another func- tion (such as being a gamete). Thus a sponge cell can dedifferentiate and redifferentiate. Cells of all other animals are organized into tissues, each of which is characterized by cells of particular mor- phology and capability. But their competence to dedifferentiate prevents sponge cells from forming clearly defined tissues (and therefore, of course, organs, which are composed of tissues). Because cells differentiate irreversibly in all animals ex- cept sponges, scientists infer that bodies containing cells spe- cialized to serve particular functions have an advantage compared to those with cells that potentially have multiple functions. Judging by the relative diversity of animals with spe- cialized tissues and those lacking them, tissues are a favorable adaptation. Presumably the advantage to the animal is embod- ied in the old adage “Jack of all trades, master of none.”
A body cavity made possible the development
of advanced organ systems
In the process of embryonic development, the cells of animals of most groups organize into three layers (called germ layers): an outer ectoderm, an inner endoderm, and an intermediate mesoderm. Animals with three embryonic cell layers are said to be triploblastic. Part of the maturation from the embryo is that certain organs and organ systems develop from each germ layer. The ecto- derm gives rise to the outer covering of the body and the nervous system; the endoderm gives rise to the digestive system, including the intestine; and the skeleton and muscles develop from the meso- derm. Cnidarians have only two layers (thus they are diplobastic), the endoderm and the ectoderm, and lack organs. Sponges lack germ layers altogether; they, of course, have no tissues or organs. All triploblastic animals are members of the Bilateria.
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Spiralian Protostomes
Radial cleavage
Blastopore becomes mouth
Blastopore becomes anus Archenteron
Mesoderm
Archenteron
Mesoderm Development arrested
Determinate development
Indeterminate development
Normal embryos
Spiral cleavage
Side view Top view
Axis
Cleavage (^) Embryonic CellsFate of BlastoporeFate of Formation ofCoelom
Four-cell embryo
Four-cell embryo
Deuterostomes
Coelom
Mouth
Anus
Side view Top view
Axis
Cell excised
Cell excised
Figure 32.3 Embryonic development in protostomes and deuterostomes. In spiralian protostomes, embryonic cells cleave in
a spiral pattern and exhibit determinate development; the blastopore becomes the animal’s mouth, and the coelom originates from a split among endodermal cells. In deuterostomes, embryonic cells cleave radially and exhibit indeterminate development; the blastopore becomes the animal’s anus, and the coelom originates from an invagination of the archenteron.
early in development, but that may have specialized functions. Development of segmentation is mediated at the molecular level by Hox genes (see chapters 19 and 25, and in section 32.4). During early development, segments first are obvious in the mesoderm but later are reflected in the ectoderm and endoderm. Two advantages result from early embryonic segmentation:
1. In highly segmental animals, such as earthworms (phylum Annelida), each segment may develop a more or less complete set of adult organ systems. Because these are redundant systems, damage to any one segment need not be fatal because other segments duplicate the damaged segment’s functions. 2. Locomotion is more efficient when individual segments can move semi-independently. Because partitions isolate the segments, each can contract or expand autonomously. Therefore, a long body can move in ways that are often quite complex. Segmentation underlies the organization of body plans of the most morphologically complex animals. In some adult
Formation of the coelom
The coelom arises within the mesoderm. In protostomes, cells simply move apart from one another to create an expanding coelomic cavity within the mass of mesodermal cells. In deu- terostomes, groups of cells pouch off the end of the archenteron, which you will recall is the primitive gut—the hollow in the center of the developing embryo that is lined with endoderm. The consistency of deuterostome development and its distinctiveness from that of the protostomes suggest that it evolved once, in the ancestor of the deuterostome phyla. The mode of development in protostomes is more diverse, but be- cause of the distinctiveness of spiral development, scientists infer it also evolved once, in the common ancestor to all spiralian phyla.
Segmentation allowed for redundant systems
and improved locomotion
Segmented animals consist of a series of linearly arrayed com- partments that typically look alike (see figure 34.14), at least
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arthropods, the segments are fused, but segmentation is usu- ally apparent in embryological development. In vertebrates, the backbone and muscle blocks are segmented, although seg- mentation is often disguised in the adult form. Previously, zoologists considered that true segmentation was found only in annelids, arthropods, and chordates, but seg- mentation is now recognized to be more widespread. Animals such as onychophorans (velvet worms), tardigrades (water bears), and kinorhynchs (mud dragons) are also segmented.
Learning Outcomes Review 32.
Animals are distinguished on the basis of symmetry, tissues, type of body cavity, sequence of embryonic development, and segmentation. A pseudocoelom is a space that develops between the mesoderm and endoderm; a coelom develops entirely within mesoderm. In bilaterians, protostomes develop the mouth prior to the anus; deuterostomes develop the mouth after the anus has formed. Segmentation allows redundant systems and more efficient locomotion.
■ How is cephalization related to body symmetry?
32.3 The Classification of Animals
Learning Outcomes
1. List the major criteria scientists have used to distinguish **animal phyla.
- Distinguish between spiralian and ecdysozoan** **organisms.
- Identify the placement of humans among the** animal phyla.
Multicellular animals, or metazoans, are traditionally divided into 35 to 40 phyla (singular, phylum ). There is little disagree- ment among biologists about the placement of most animals in phyla, although zoologists disagree on the status of some, par- ticularly those with few members or recently discovered ones. The diversity of animals is obvious in tables 32.2 and 32.3, which describe key characteristics of 20 of the phyla. Traditionally, the phylogeny of animals has been in- ferred using features of anatomy and aspects of embryological development, as discussed earlier, from which a broad consen- sus emerged over the last century concerning the main branches of the animal tree of life. In the past 30 years, data derived from molecular features have been added, leading to some rethinking of classification schemes. Depending on the features compared, biologists may draw quite different family trees—although, of course, there is only one way that evolu- tion actually occurred, and the goal of phylogeny is to detect that history. Whether morphological or molecular characters (or both) are used, the underlying principle is the same: system- atists use features they assume to have evolved only once, so the animals sharing such a feature are inferred to be more closely
related to one another than they are to animals not sharing the feature. The shared derived characters unique to a group and its ancestors define a monophyletic assemblage termed a clade (see chapter 23). The animal phylogenetic tree viewed in these terms is a hierarchy of clades nested within larger clades, and containing smaller clades.
Tissues and symmetry separate the Parazoa and Eumetazoa Systematists traditionally divided the kingdom Animalia (also termed Metazoa) into two main branches. Parazoa (“near ani- mals”) comprises animals that, for the most part, lack definite symmetry, and that do not possess tissues. These are the sponges, phylum Porifera. Because they are so different in so many ways from other animals, some scientists inferred that sponges were not closely related to other animals, which would mean that what we consider animals had two separate origins. Eumetazoa (“true animals”) are animals that have a definite shape and symmetry. All have tissues, and most have organs and organ systems. Now most systematists agree that Parazoa and Eumetazoa are descended from a common ancestor, so animal life had a single origin. And although most trees constructed including molecular data consider Parazoa to be at the base of the animal tree of life, some do not. Further divisions are based on other key features, as dis- cussed previously. Bilaterally symmetrical animals (which are also triploblastic) are divided into the groups Protostomia and Deuterostomia depending on whether the embryonic blasto- pore (see figure 32.3) becomes the mouth or the anus (or both), respectively, in the adult animal. Animals are traditionally classified into 35 to 40 phyla. The evolutionary relationships among the animal phyla are based on the inference that phyla sharing certain fundamental morphological and molecular characters are more closely re- lated to one another than they are to phyla not sharing those characters. Phylogenetically informative characters are inferred to have arisen only once.
Molecular data help reveal evolutionary relationships Gene sequence data are accumulating at an accelerating pace for all animal groups. Phylogenies developed from different molecules sometimes suggest quite different evolutionary rela- tionships among the same groups of animals. However, com- bining data from multiple genes has resolved the relationships of most phyla. Current studies are using sequences from hun- dreds of genes to try to fully resolve the animal tree of life. Molecular data are helping to resolve some problems with the traditional phylogeny, such as puzzling groups that did not fit well into the widely accepted phylogeny. These data may be especially helpful in clarifying relationships that conven- tional data cannot, as, for example, in animals such as parasites. Through dependence on their host, the anatomy, physiology, and behavior of parasites tends to be greatly altered, so features that may reveal the phylogenetic affinities of free-living ani- mals can be highly modified or lost.
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T A B L E 3 2. 3
Some Important Animal Phyla with Fewer Species—and Three Recently
Discovered Ones
Phylum Typical Examples Key Characteristics
Approximate Number of Named Species
Rotifera (wheel animals) Rotifers Small pseudocoelomates with a complete digestive tract including a set of complex jaws. Cilia at the anterior end beat so they resemble a revolving wheel. Some are very important in marine and freshwater habitats as food for predators such as fishes.
2000
Nemertea (ribbon worms) (also called Rhynchocoela)
Lineus Protostome worms notable for their fragility—when disturbed, they fragment in pieces. Long, extensible proboscis occupies a coelomic space; that of some tipped by a spearlike stylet. Most marine, but some live in fresh water, and a few are terrestrial.
900
Tardigrada (water bears) Hypsibius Microscopic protostomes with five body segments and four pairs of clawed legs. An individual lives a week or less but can enter a state of suspended animation (“cryptobiosis”) in which it can survive for many decades. Occupy marine, freshwater, and terrestrial habitats.
800
Brachiopoda (lamp shells)
Lingula Protostomous animals encased in two shells that are oriented with respect to the body differently than in bivalved mollusks. A ring of ciliated tentacles (lophophore) surrounds the mouth. More than 30,000 fossil species are known.
300
Onychophora (velvet worms)
Peripatus Segmented protostomous worms resembling tardigrades; with a chitinous soft exoskeleton and unsegmented appendages. Related to arthropods. The only exclusively terrestrial phylum, but what are interpreted as their Cambrian ancestors were marine.
110
Ctenophora (sea walnuts)
Comb jellies, sea walnuts
Gelatinous, almost transparent, often bioluminescent marine animals; eight bands of cilia; largest animals that use cilia for locomotion; complete digestive tract with anal pore.
100
Chaetognatha (arrow worms)
Sagitta Small, bilaterally symmetrical, transparent marine worms with a fin along each side, powerful bristly jaws, and lateral nerve cords. Some inject toxin into prey and some have large eyes. It is uncertain if they are coelomates, and, if so, whether protostomes or deuterostomes.
100
Loricifera (loriciferans)
Nanaloricus mysticus Tiny marine pseudocoelomates that live in spaces between grains of sand. The mouth is borne on the tip of a flexible tube. Discovered in 1983.
10
Cycliophora (cycliophorans) Symbion Microscopic animals that live on mouthparts of claw lobsters. Discovered in 1995.
3
Micrognathozoa (micrognathozoans)
Limnognathia Microscopic animals with complicated jaws. Discovered in 2000 in Greenland.
1
642 part V Diversity of Life on Earth
Morphology- and molecule-based phylogenies
agree on many major groupings
Although they differ from one another in some respects, phy- logenies incorporating molecular data or based entirely on them share some deep structure with the traditional animal tree of life. Figure 32.4 is a summary of animal phylogeny developed from morphological, molecular, life-history, and other types of relevant data. Some aspects of this view have been contradicted by studies based on particular characters or using particular analytical methods. It is an exciting time to be a systematist, but shifts in understanding of relationships among groups of animals can be frustrating to some! Like any scientific idea, a phylogeny is a hypothesis, open to challenge and to being revised in light of additional data. One consistent result is that Porifera (sponges) consti- tutes a monophyletic group that shares a common ancestor with other animals. Some systematists had considered sponges to comprise two (or three) groups that are not particularly closely related, but molecular data support what had been the majority view, that phylum Porifera is monophyletic. And, as mentioned earlier, all animals are found to be monophyletic. Among eumetazoans, molecular data are in accord with the traditional view that cnidarians (hydras, sea jellies, and cor- als) branch off the tree before the origin of animals with bilat- eral symmetry. Our understanding of the phylogeny of the deuterostome branch of Bilateria (discussed in chapter 34 ) has not changed much, but our understanding of the phylogeny of protostomes has been altered by molecular data. Most revolutionary is that annelids and arthropods, which had been considered closely related based on the occurrence of segmentation in both, belong to separate clades. Now arthro- pods are grouped with protostomes that molt their cuticles at least once during their life. These are termed ecdysozoans, which means “molting animals” (see chapter 34). Molecular se- quence data can help test our ideas of which morphological fea- tures reveal evolutionary relationships best; in this case, molecular data allowed us to see that, contrary to our hypoth- esis, segmentation seems to have evolved convergently, but molting did not. But not all features are easy to diagnose, and molecular data do not resolve all uncertainties. The enigmatic phylum Ctenophora (comb jellies)—pronounced with a silent C—has been considered both diploblastic and triploblastic and has been thought to have both a complete gut and a blind gut. Likewise the enigmatic phylum Chaetognatha (arrow worms) has been considered both coelomate and pseudocoelomate, and if coelomate, both protostome and deuterostome (as reflected in figure 32.4). Their placement in phylogenies varies, seeming to depend on the features and methods used to construct the tree. Further research is needed to resolve these uncertainties. Molecular data are contributing to our understanding of relationships among animal phyla. Animals are monophyletic, as are sponges—relationships that were uncertain using only morphological data. Molecular data confirm that cnidarians branched off from the rest of animals before bilaterial symme- try evolved. Although the position of ctenophores has not been
resolved, molecular data have significantly altered some ideas of protostome evolution.
Morphology-based phylogeny focused on the state of the coelom In the morphology-based animal family tree, bilaterally sym- metrical animals comprised three major branches. If the body has no cavity (other than the gut), the animal is said to be acoe- lomate; members of phylum Platyhelminthes are acoelomates. A body cavity not lined with tissue derived from meso- derm is a pseudocoelom; members of the phylum Nematoda are pseudocoelomate. A body cavity lined with tissue derived from mesoderm is a coelom; we and members of the phylum Annelida are coelomate. All acoelomates and pseudocoelomates are protostomes; some coelomates are protostomes and some are deuterostomes.
Protostomes consist of spiralians and ecdysozoans Two major clades of protostomes are recognized as having evolved independently since ancient times: the spiralians and the ecdysozoans (see figure 32.4).
Spiralia
Spiralian animals grow by gradual addition of mass to the body. Most live in water, and propel themselves through it using cilia or contractions of the body musculature. Spiralians undergo spiral cleavage (see figure 32.3). There are two main groups of spiralians: Lophotrochozoa and Platyzoa. Lophotrochozoa includes most coelomate pro- tostome phyla; those animals move by muscular contractions. Most platyzoans are acoelomates; these animals are tiny or flat, and move by ciliary action. Some platyzoans (such as rotifers, gnathostomulids, and the recently discovered phylum Micro- gnathozoa have a set of complicated jaws. The most prominent group is phylum Platyhelminthes; a flatworm has a simple body with no circulatory or respiratory system but a complex repro- ductive system. This group includes marine and freshwater pla- narians as well as the parasitic flukes and tapeworms. Lophotrochozoa consists of two major phyla and several smaller ones. Many of the animals have a type of free-living larva known as a trochophore, and some have a feeding struc- ture termed a lophophore, a horseshoe-shaped crown of cili- ated tentacles around the mouth used in filter-feeding. The phyla characterized by a lophophore are Bryozoa and Brachio- poda. Lophophorate animals are sessile (anchored in place). Among the lophotrochozoans with a trochophore are phyla Mollusca and Annelida. Mollusks are unsegmented, and their coelom is reduced to a hemocoel (open circulatory space) and some other small body spaces. This phylum includes ani- mals as diverse as octopuses, snails, and clams. Annelids are segmented coelomate worms, the most familiar of which is the earthworm, but also includes leeches and the largely ma- rine polychaetes.
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NematodaTardigradaArthropodaOnychophoraChaetognathaEchinodermataChordata
Ecdysozoa
Deuterostomes
32.4 The Roots of the Animal
Tree of Life
Learning Outcomes
1. Explain the colonial flagellate hypothesis of metazoan **origin and why it is now favored.
- Describe the possible role ofHox genes in the Cambrian** explosion.
Some of the most exciting contributions of molecular sys- tematics are being made to our understanding of the base of the animal family tree—the origins of the major clades of animals.
Metazoans appear to have evolved from colonial protists The ancestor to all animals was presumably a protist (see chapter 29), but it is not clear from which line of protists animals evolved. Evidence is available to support two major hypotheses. ■ The multinucleate hypothesis is that metazoans arose from a multinuclear protist similar to today’s ciliates. Each nucleus became compartmentalized into a cell, resulting in the multicellular condition. ■ The colonial flagellate hypothesis, first proposed by Ernst Haeckel in 1874, is that metazoans descended from colonial protists. Each colony is a hollow sphere composed of flagellated cells. Some of the cells of sponges are strikingly like those of choanoflagellate protists. Molecular data based on ribosomal RNA sequences favor the colonial flagellate hypothesis, and reject the multinucleate cili- ate hypothesis based on evidence that metazoans are more closely related to eukaryotic algae than to ciliates.
Molecular analysis may explain the Cambrian explosion Most major animal body plans can be seen in fossils of Cam- brian age, dating from 543 to 525 mya. Although fossil cnidar- ians are found in rocks from the Ediacaran period, as old as 565 million years, along with what appear to be fossil mollusks and the burrows of worms, the great diversity of animals evolved quite rapidly in geological terms around the beginning of the Cambrian period—an event known as the Cambrian explosion. Biologists have long debated what caused this enor- mous expansion of animal diversity (figure 32.6). Many have argued that the emergence of new body plans was biological— the consequence of the evolution of predation, which encour- aged an arms race between defenses, such as armor, and innovations that improved mobility and hunting success. Oth- ers have attributed the rapid diversification in body plans to physical factors—such as the build-up of dissolved oxygen and minerals in the oceans.
plants or animals. It is said that if everything in the world ex- cept nematodes were to disappear, an outline of what had been there would be visible in the remaining nematodes!
Deuterostomes include chordates
and echinoderms
Deuterostomes consist of fewer phyla and species than proto- stomes, and are more uniform in many ways, despite great dif- ferences in appearance. Echinoderms such as sea stars, and chordates such as humans, share a mode of development that is evidence of their evolution from a common ancestor, and sepa- rates them clearly from other animals.
Learning Outcomes Review 32.
Scientists have defined phyla based on tissues, symmetry, characteristics such as presence or absence of a coelom or pseudocoelom, protostome versus deuterostome development, growth pattern and larval stages, and molecular data. Among protostomes, spiralian organisms have a growth pattern in which their body size simply increases; ecdysozoans must molt in order to grow larger. Among the deuterostome phyla are Echinodermata and Chordata, which includes humans.
■ Why do systematists attempt to characterize each group of animals by one or more features that have evolved only once?
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A circulatory system is an example of a specialized organ system that assists with distribution of nutrients and removal of wastes. Bilaterians have two main types of development. In a protostome, the mouth develops from or near the blastopore. A protostome has determinate development, and many have spiral cleavage. In a deuterostome, the anus develops from the blastopore. A deuterostome has indeterminate development and radial cleavage. Segmentation allowed for redundant systems and improved locomotion. Segmentation, which evolved multiple times, allows for efficient and fl exible movement because each segment can move somewhat independently. Another advantage to segmentation is redundant organ systems.
32.3 The Classification of Animals
Animals are classifi ed into 35 to 40 phyla based on shared characteristics. Systematists attempt to use features assumed to have evolved only once. Tissues and symmetry separate the Parazoa and Eumetazoa. With the exception of sponges, animals exhibit embryonic germ layers and differentiated cells that form tissues. These characteristics lead to most animals being termed collectively the Eumetazoa; other animals, including the sponges, are Parazoa.
32.1 Some General Features of Animals
Features common to all animals are multicelluarity, heterotrophic lifestyle, and lack of a cell wall. Other features include specialized tissues, ability to move, and sexual reproduction.
32.2 Evolution of the Animal Body Plan
Most animals exhibit radial or bilateral symmetry. Most sponges are asymmetrical, but other animals are bilaterally or radially symmetrical at some time during their life. The body parts of radially symmetrical animals are arranged around a central axis. The body of a bilaterally symmetrical animal has left and right halves. Most bilaterally symmetrical organisms are cephalized and can move directionally.
The evolution of tissues allowed for specialized structures and functions. Each tissue consists of differentiated cells that have characteristic forms and functions.
A body cavity made possible the development of advanced organ systems. Most bilaterian animals possess a body cavity other than the gut. A coelom is a cavity that lies within tissues derived from mesoderm. A pseudocoelom lies between tissues derived from mesoderm and the gut (which develops from endoderm). The acoelomate condition and the pseudocoelom appear to have evolved more than once, but the coelom evolved only once.
Chapter Review
Whether these causes or others are at the heart of the Cambrian explosion, molecular studies in the field of evolu- tionary developmental biology may provide a mechanism for the emergence of so many body plans. Much of the variation in animal body plans is associated with changes in the location or time of expression of homeobox genes ( Hox genes) in embryos (see chapters 19 and 25). Hox genes specify the identity of de- veloping body parts, such as the legs, thorax, and antennae. Per- haps the Cambrian explosion reflects the evolution of the Hox developmental gene complex, which provides a mechanism for producing rapid changes in body plan.
Learning Outcomes Review 32.
The hypothesis of evolution of metazoans from colonial flagellates is favored because of the similarity between flagellate colonies and metazoan sponges, and because of molecular data based on ribosomal RNA sequences. Animal fossils become highly abundant in the Cambrian period in what is known as the Cambrian explosion. Hox genes, which control development of body shape and parts, may be responsible for the diversity found in this period.
■ What alternative interpretations are there for the fossils that have led to the idea of the Cambrian explosion?
Figure 32.6 Diversity of animals that evolved during
the Cambrian explosion. The Cambrian saw an astonishing variety of body plans, many of which gave rise to the animals we fi nd today. The natural history of these species is open to speculation.
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Blastopore becomes mouth
Archenteron
Mesoderm
A P P L Y
- The following diagram is of the blastopore stage of embryonic development. Based on the information in the diagram, which of the following statements is correct? a. It is a diagram of a protostome. b. It would have formed by radial cleavage. c. It would exhibit determinate development. d. All of the above are correct.
- Which of the following characteristics would not apply to a species in the Ecdysozoa? a. Bilateral b. Indeterminate cleavage c. Molt at least once in their life cycle d. Metazoan
- In the rain forest you discover a new species that is terrestrial, has determinate development, molts during its lifetime, and
possesses jointed appendages. To which phylum of animals should it be assigned?
S Y N T H E S I Z E
- Worm evolution represents an excellent means of understanding the evolution of a body cavity. Using the phyla of worms Nematoda, Annelida, Platyhelminthes, and Nemetera, construct a phylogenetic tree based only on the form of body cavity (refer to figure 32.2 and table 32.2 for assistance). How does this relate to the material in figure 32.4? Should body cavity be used as the sole characteristic for classifying a worm?
- Most students find it hard to believe that Echinodermata and Chordata are closely related phyla. If it were not for how their members form a body cavity, where would you place Echinodermata in the animal kingdom? Defend your answer.
O N L I N E R E S O U R C E
www.ravenbiology.com Understand, Apply, and Synthesize—enhance your study with animations that bring concepts to life and practice tests to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.
648 part V Diversity of Life on Earth
