r:~~~j~;a~~~ulticellular,rcastilho.pt/da/calendario_files/cap_32_campbell.pdfthe resulting...

.... Figure 32.1 Which of these organisms are animals?

KEY CONCEPTS

32.1 Animals are multicellular, heterotrophiceukaryotes with tissues that develop fromembryonic layers

32.2 The history of animals spans more than half abillion years

32.3 Animals can be characterized by Ilbody plans"32.4 New views of animal phylogeny are emerging

from molecular data

Reading the last few chapters, you may have felt like atourist among some unfamiliar organisms, such asslime molds, whisk ferns, and sac fungi. You proba~

bly are more at home with the topic introduced in thischapter-the animal kingdom, which of course includes

yourself. But animal diversity extends far beyond humansand the dogs, cats, birds, and other animals we humans reg·ularly encounter. For example, the diverse organisms inFigure 32.1 are all animals, including those that appear toresemble lacy branches, thick stems, and curly leaves. Todate biologists have identified 1.3 million extant (living)species of animals. Estimates of the actual number of animal species run far higher. This vast diversity encompassesa spectacular range of morphological variation, from coralsto cockroaches to crocodiles.

In this chapter, we embark on a tour ofthe animal kingdomthat will continue in the next two chapters. We will considerthe characteristics that all animals share, as well as those that

distinguish various taxonomic groups. This information iscentral to understanding animal phylogeny, a topic that is cur·rently a lively arena of biological research and debate. as youwill read.

654

r:~~~J~;a~~~ulticellular,heterotrophic eukaryotes withtissues that develop fromembryonic layers

Constructing a good definition of an animal is not straightforward, as there are exceptions to nearly every criterion for distinguishing animals from other life~forms. However, severalcharacteristics of animals, when taken together, sufficientlydefine the group for our discussion.

Nutritional Mode

Animals differ from both plants and fungi in their mode of nu~trition. Plants are autotrophic eukaryotes capable ofgenerating

organic molecules through photosynthesis. Fungi are het·erotrophs that grow on or near their food and that feed by ab·sorption (often after they have released enzymes that digest thefood outside their bodies). Unlike plants, animals cannot con

struct all of their own organic molecules and so, in most cases.they ingest them-either by eating other living organisms or byeating nonliving organic material. But unlike fungi, most animals do not feed by absorption; instead, animals ingest theirfood and then use enzymes to digest it within their bodies.

Cell Structure and Specialization

Animals are eukaryotes, and like plants and most fungi (butunlike most protists), animals are multicellular. In contrast toplants and fungi, however, animals lack the structural supportofcell walls. Instead, animal cells are held together by structuralproteins, the most abundant being collagen (see Figure 6.30),which is found only in animals.

Many animals have two types ofspecialized cells not seen inother multicellular organisms: muscle cells and nerve cells. Inmost animals, these cells are organized into muscle tissue andnervous tissue, respectively, and are responsible for moving thebody and conducting nerve impulses. The ability to move andconduct nerve impulses underlies many ofthe adaptations thatdifferentiate animals from plants and fungi, making muscleand nerve cells central to what it means to be an animal.

OThe zygote of an animalundergoes a series of L_(-mitotic cell divisions I

called cleavage.

Zygote

"","Cleavage

... Figure 32.2 Early embryonic development in animals.

Archenteron

Blastocoel

Blastocoel

Cross sectionof blastula

Blastula

Eight-cell stage

"","Cleavage

"The pouch formedby gastrulation, called the l __j,~-Sarchenteron, opens to the r "","';~Endoderm

outside via the blastopore.Edoderm

oMost animals alsoundergo gastrulation, a

process in which one end ofthe embryo folds inward,

eKpands, and eventually fillsthe blastocoel, producing

layers of embryonic tissues:the ectoderm (outer layer)

and the endoderm(inner layer).

f) An eight-cell embryo L_t-I'-_--'t'isformed by three I

rounds of cell division.

o In most animals, cleavageresults in the formation ofa multicellular stage called

a blastula. The blastula istypically a hollow ball of

cells that surround a cavitycalled the blastocoel.

oThe endoderm of thearchenteron develops

into the tissuelining the animal's

digestive tract.

Reproduction and Development

Most animals reproduce sexually, and the diploid stage usuallydominates the life cycle. In most spe<ies, a small, flagellatedsperm fertilizes a larger, nonmotile egg, forming a diploid zygote.The zygote then undergoes cleavage, a succession of mitotic ceUdivisions without cell growth bern'een division cycles. During thedevelopment of most animals, cleavage leads to the formation ofa multicellular stage called a blastula, which in many animalstakes the form ofa hollow ball (Figure 32.2). Following the blastula stage is the process of gastrulation, during which layers ofembryonic tissues that will develop into adult body parts are produced. The resulting developmental stage is called a gastrula.

Some animals, including humans, develop directly intoadults through transient stages of maturation, but the life cycles of many animals also include at least one larval stage. Alarva is a sexually immature form ofan animal that is morphologically distinct from the adult, usually eats different food,and may even have a different habitat than the adult, as in thecase of the aquatic larva of a mosquito or a dragonfly. Animallarvae eventually undergo metamorphosis, a developmentaltransformation that turns the animal into a juvenile, which resembles an adult but is not yet sexually mature.

Although adult animals vary widely in morphology, the underlying genetic nern'ork that controls animal development is similar across a broad range of taxa. All eukaryotes have genes thatregulate the expression ofother genes, and many ofthese regulatory genes contain common sets of DNA sequences calledhomeoboxes (see ampter 21). Animals shareaunique homeoboxcontaining famUy ofgenes, knov>'I1 as Hox genes. Hox genes playimportant roles in the development of animal embryos, controlling the expression ofdozens or even hundreds ofother genes thatinfluence animal morphology (see Chapter 25).

Sponges, which are among the simplest extant animals, haveHox genes that regulate formation of water channels in the bodywall, a key feature ofsponge morphology (see Chapter 33). In theancestors of more complex animals, the Hox gene family underwent further duplications, yielding a more versatile "toolkit" forregulating development. In vertebrates, insects, and most otheranimals, Hox genes regulate patterning ofthe anterior-posterior(front-to-back) axis, as well as other aspects ofdevelopment. Thesame conserved genetic nety,'ork governs the development ofboth a flyand a human, despite their obvious differences and hWldreds of millions of years of divergent evolution.

CHAPTER THIRTY·TWO An Introduction to Animal Diversity 655

1. Summarize the main steps of animal development.\Vhat family ofcontrol genes plays a major role?

2. _'*,.014 What animal characteristics would be

needed by an imaginary plant that could chase, capture, and digest its prey-yet could also extract nutri

ents from soil and conduct photosynthesis?

For suggested answers, see Appendix A.

CONCEPT CHECI( 32.1 nation of morphological and molecular evidence indicates thatchoanoflagellates are among the closest living relatives of animals. Based on such results, researchers hypothesize that thecommon ancestor ofliving animals may have been a stationary

suspension feeder, similar to present-day choanoflagellates. In

this section, we will survey the fossil evidence for how animalsevolved from their distant common ancestor over four geo·logie eras (see Table 25.1 to review the geologic time scale.)

r;~:'~i:~~2~:animals spansmore than half a billion years

The animal kingdom includes not only the great diversity ofliving species, but also the even greater diversity of extinctones. (Some paleontologists have estimated that 99% of all animal species are extinct.) Various studies suggest that thisgreat diversity has its origins in evolutionary changes that occurred during the last billion years. For example, some estimates based on molecular clocks suggest that the ancestors ofanimals diverged from the ancestors of fungi about a billionyears ago. Similar studies suggest that the common ancestorof living animals may have lived sometime between 675and 875 million years ago.

To learn what this common ancestor may have been like,

scientists have sought to identify protist groups that areclosely related to animals. As shown in Figure 32.3, a combi·

Neoproterozoic Era(1 Billion-542 Million Years Ago)

Despite the molecular data indicating an earlier origin of animals, the first generally accepted macroscopic fossils of animals range in age from 565 to 550 million years old. Thesefossils are members of an early group of multicellular eukaryotes, known collectively as the Ediacaran biota. These soft

bodied organisms were named for the Ediacara Hills ofAustralia, where they were first discovered (Figure 32,4).

Similar fossils have since been found on other continents.Some are sponges, while others may be related to livingcnidarians. Still others of these fossil organisms have proveddifficult to classify, as they do not seem to be closely related toany living animal or plant groups.

In addition to these macroscopic fossils, Neoproterozoicrocks have also yielded what may be microscopic signs of earlyanimals. As you read in Chapter 25, 575-million-year-old microfossils discovered in China appear to exhibit the basic structural organization of present-day animal embryos. However,

debate continues about whether the fossil embryos are animals

oMorphologically,choanoflagetlate cellsand the collar cells (or

__- Ilchoanocytes) ofsponges are almostindistinguishable.

Collar cell(choanocyte)

f) Similar collar cells have been identified in other animals, includingcnidarians, flatworms, and echinoderms-but they have never beenobserved in non-choanoflagellate protists or in plants or fungi.

)

Other animals

Sponges

Choonoflagellates

•,3•,;

OTHEREUKARYOTES

oDNA sequence data indicate that choanoflageliatesand animals are sister groups, In addition, signalingand adhesion genes previously known only fromanimals have been discovered in choanoflagellates.

'f Figure 32.3 Three lines of evidence thatchoanoflagellates are closely related to animals.I!I Are the data described in 0 consistent with.. predictions that could be made from the evidence

in 0 and 8? Explain.

656 UNIT fiVE The Evolutionary History of Biological Diversity

Paleozoic Era (542-251 Million Years Ago)

... Figure 32.4 Ediacaran fossils. Fossils dating from 565-550million years ago include animals (al with simple. radial forms and(bl with many body segments,

... Figure 32.5 ACambrian seascape. This artist's reconstructiondepicts a diverse array of organisms found in fossils from the BurgessShale site in British Columbia. Canada, The animals include Pikaia(swimming eel-like chordate). Hallucigenia (animal with toothpick-likespikes). Anomalocaris (large animal with anterior grasping limbs and acircular mouth). and Marella (arthropod swimming at left).

(b) Spriggina floundersi

or are members ofextinct groups that are closely related to animals (but are not actually animals). Though older fossils ofanimals may be discovered in the future, the fossil record as it isknown today strongly suggests that the end of the Neoproterozoic era was a time of increasing animal diversity.

(a) Mawsonites spriggi

Animal diversification appears to have accelerated dramatically from 535 to 525 million years ago, during the Cambrianperiod of the Paleozoic era-a phenomenon often referred toas the Cambrian explosion (see Chapter 25). In strata formedbefore the Cambrian explosion, only a few animal phyla can berecognized. But in strata that are 535 to 525 million years old,paleontologists have found the oldest fossils ofabout half of allextant animal phyla, including the first arthropods, chordates,and echinoderms. Many of these distinctive fossils-which include the first animals with hard mineralized skeletons-lookquite different from most living animals (Figure 32.5). But forthe most part, paleontologists have established that theseCambrian fossils are members of extant animal phyla-or atleast are close relatives.

What caused the Cambrian explosion? There are several current hypotheses. Some evidencesuggests that new predator-preyrelationships that emerged in the Cambrian periodgenerateddiversitythrough natural selection. Predators acquired novel adaptations, such as forms of locomotion that helped them catchprey, while prey species acquired new defenses, such as protective shells. Another hypothesis focuses on a rise in atmosphericoxygen that preceded the Cambrian explosion. Greater oxygenavailability would have provided opportunities for animals withhigher metabolic rates and larger body sizes to thrive. Athird hypothesis proposes that the evolution of the Hox gene complexprovided the developmental flexibility that resulted in variationsin morphology. These hypotheses are not mutually exclusive,however; predator-prey relationships, atmospheric changes, anddevelopmental flexibility may each have played a role.

The Cambrian period was followed by the Ordovician, Silurian, and Devonian periods, when animal diversity continuedto increase, although punctuated by episodesofmass extinctions(see Figure 25.14). Vertebrates (fishes) emerged as the top predators of the marine food. web. By 460 million years ago, groupsthat diversified during the Cambrian period were making an impact on land. Arthropods began to adapt to terrestrial habitats,as indicated by the appearance of millipedes and centipedes.Fern galls-enlarged cavities that resident insects stimulate fernplants to form, providing protection for the insects-date backat least 302 million years, suggesting that insects and plants wereinfluencing each other's evolution by that time.

Vertebrates made the transition to land around 360 millionyears ago and diversified into numerous terrestrial groups.Two of these survive today: the amphibians (such as frogs andsalamanders) and the amniotes (reptiles and mammals). Wewill explore these groups, known collectively as the tetrapods,in more detail in Chapter 34.

Mesozoic Era (251--J;5.5 Million Years Ago)

No fundamentally new animal groups emerged during theMesozoic era. But the animal phyla that had evolved during thePaleozoic now began to spread Into new ecological habitats. Inthe oceans, the first coral reefs formed, providingother animalswith new marine habitats. Some reptiles returned to the waterand succeeded as large aquatic predators. On land, descentwith modification in some tetrapods led to the origin of wingsand other flight equipment in pterosaurs and birds. Large and


Cenozoic Era(65.5 Million Years Ago to the Present)

small dinosaurs emerged, both as predators and herbivores. Atthe same time, the first mammals-tiny nocturnal insecteaters-appeared on the scene. In addition, as you read inChapter 30, flowering plants (angiosperms) and insects bothunderwent dramatic diversifications during the late Mesozoic.

rZ~~:Jz c~~·:e characterized by"body plans"

Site ofgastrulation

Site ofgastrulation

I~

RESULTS

Did fl-catenin play an ancient role in themolecular control of gastrulation?

EXPERIMENT In most animals. gastrulation leads to the formation of three layers of embryonic cells. In several species, theprotein l3-catenin marks the site of gastrulation and activatesthe transcription of genes necessary for gastrulation. AthulaWikramanayake and Mark Martindale, of the University ofHawaii, and colleagues tested whether l3-catenin also helps tocontrol gastrulation in the sea anemone Nemarosrella vecrensis.This species is in the phylum Cnidaria, a group that predates theorigin of animals whose embryos form three layers of cells.

o In early stages ofdevelopment, ~-catenin

(here labeled with greenfluorescent protein) isfound throughout theN. vectensis embryo

e In the early gastrula stage,~-catenin activity {herestained a darker redj--occurs in the inner layerof cells

o In embryos in which ~catenin activity is blocked(by a protein that binds to~-cateninl. gastrulationdoes not occur.

e By the 32-cell stage,~-catenin is concentratedon the side of the embryowhere gastrulation willoccur.

~Inui

32.2CONCEPT CHECI(

J. Put the following milestones in animal evolution inchronological oTder from oldest to most recent: (a)origin of mammals, (b) earliest evidence of terrestrialarthropods, (c) Ediacaran fauna, (d) extinction oflarge, nonflying dinosaurs.

2. _IWUI. Suppose the most recent common ancestor of fungi and animals lived 1 billion years ago. Ifthe first fungi lived 990 million years ago, would animals also have been alive at that time? Explain.

For suggested answers. see Appendix A

Mass extinctions of both terrestrial and marine animals usheredin a new era, the Cenozoic. Among the groups ofspecies thatdisappeared were the large, nonflying dinosaurs and the marinereptiles. The fossil record of the early Cenozoic documents therise of large mammalian herbivores and predators as mammalsbegan to exploit the vacated ecological niches. The global climategradually cooled throughout the Cenozoic, triggering significantshifts in many animal lineages. Among primates, for example,some species in Africa adapted to the open woodlands and savannas that replaced the former dense forests. The ancestors ofour own species were among those grassland apes.

SOURCE

Although animal species vary tremendously in morphology,their great diversity in form can be categorized into a relativelysmall number of major "body plans.~ A body plan is a set ofmorphological and developmental traits, integrated into afunctional whole-the living animal. (Note that the term planhere is not meant to imply that animal forms are the result ofconscious planning or invention.)

Like all features of organisms, animal body plans haveevolved over time. Some of the evolutionary changes appear tohave occurred early in the history ofanimal life. For example, recent research suggests that a key step in the molecular controlofgastrulation has remained unchanged for more than 500 million years (Figure 32.6). Thisearlyevolutionary innovation wasof fundamental importance: Gastrulation explains why most

CONCLUSION In N. vectensis,l3-catenin helps to determinethe site of gastrulation and is required for gastrulation to occurThe fossil record indicates that cnidarians diverged more than 500million years ago from other species in which l3·catenin is knownto influence gastrulation, suggesting that l3-catenin played anancient role in the molecular control of gastrulation,

A. rI, Wikr~lI1<Inay~ke et ~I.. An ~ncient role lor nuclear~'Ciltenin in the e\lolullon 01 ~..~I poli!nty il/1d germ layer 5egreg~llon, N<lture426:445-450 (20031.

_IIIMilIA p-catenin binds to DNA, thereby stimulating thetranscription of genes necessary for gastrulation. Based on this information, suggest a different experiment that could be used toconfirm the results in step 4, What would be the purpose of performing such an experiment?


Symmetry

(a) Radial symmetry. A radial animal. such as a sea anemone (phylumCnidaria), does not have a left side and aright side. Any imaginaryslice through the central axis divides the animal into mirror images.

Tissues

Thus, such animals have a dorsal (top) side and a ventral(bottom) side, as well as a left side and a right side and ananterior (front) end with a mouth and a posterior (back)end. Many animals with a bilaterally symmetrical body plan(such as arthropods and mammals) have sensory equipmentconcentrated at their anterior end, including a centralnervous system ("brain") in the head-an evolutionarytrend called cephalization (from the Greek kephale, head).

The symmetry of an animal generally fits its lifestyle. Manyradial animals are sessile (living attached to a substrate) orplanktonic (drifting or weakly swimming, such as jellyfish,more accurately called jellies). Their symmetry equips them tomeet the environment equally well from all sides. In contrast,bilateral animals typically move actively from place to place.Most bilateral animals have a central nervous system that enables them to coordinate the complex movements involved incrawling, burrowing, flying, or swimming. Fossil evidence indicates that these two fundamentally different kinds of symmetry have been present for at least 550 million years.

Animal body plans also vary according to the organization ofthe animal's tissues. True tissues are collections of specializedcells isolated from other tissues by membranous layers. Spongesand a few othergroups lack true tissues. In all other animals, theembryo becomes layered through the process of gastrulation(see Figure 32.2). As development progresses, these concentriclayers, called germ lLlyers, form the various tissues and organs ofthe body. Ectoderm, the germ layer covering the surface of theembryo, gives rise to the outer covering of the animal and, insome phyla, to tlle central nervous system. Endoderm, the innermost germ layer, lines the developing digestive tube, orarchenteron, and gives rise to the lining of the digestive tract(or cavity) and organs such as the liver and lungs ofvertebrates.

Animals that have only these tv.'o germ layers are said to bediploblastic. Diploblasts include the animals called cnidariansijellies and corals, for example) as well as the comb jellies (seeChapter 33). All bilaterally symmetrical animals have a thirdgerm layer, called the mesoderm, betv.'een the ectoderm andendoderm. Thus, animals with bilateral symmetryare also saidto be triploblastic (having three germ layers). In triploblasts,the mesoderm forms the muscles and most other organs between the digestive tract and the outer covering of the animal.Triploblasts include a broad range ofanimals, from flatwormsto arthropods to vertebrates. (Although some diploblasts actually do have a third germ layer, it is not nearly as well developedas the mesoderm ofanimals considered to be triploblastic.)

One very basic way that animals can be categorized is by tlle type

ofsymmetry oftheir bodies-or the absence ofsymmetry. Mostsponges, for example, lack symmetry altogether. Among the animals that do have symmetrical bodies, symmetry can take different forms. Some animals exhibit radial symmetry, the formfound in a flowerpot (Figure 32.7a). Seaanemones, for example,have a top side (where the mouth is located) and a bottom side.But they have no front and back ends and no left and right sides.

The two-sided symmetry seen in a shovel is an exampleof bilateral symmetry (Figure 32.7b). A bilateral animalhas two axes oforientation: front to back and top to bottom.

animals are not a hollow ball of cells. But other aspects ofanimal body plans may have changed many times as various animal lineages evolved and diversified. As you explore the majorfeatures of animal body plans, bear in mind that similar bodyforms may have evolved independently in two different lineages. Consider, for example, the group of invertebrate animalscalledgastropods (class Gastropoda). Thisgroup includes manyspecies that lack shells and are referred to as slugs, along withmany shelled species, such as snails. All slugs have a similarbody plan and hence belong to the same grade (a group whosemembers share key biological features). However, phylogeneticstudies show that several gastropod lineages independently losttheir shells and became slugs. As illustrated by this example, agrade is not necessarily equivalent to a clade (a group that includes an ancestral spe<ies and all of its descendants).

(b) Bilateral symmetry. A bilateral animal. such as a lobster (phylumArthropoda), has a left side and a right side. Only one imaginary cutdivides the animal into mirror'lmage halves

... Figure 32.7 Body symmetry. The flowerpot and shovel areincluded to help you remember the radial·bilateral distinction.

Body Cavities

Most triploblastic animals possess a body cavity, a fluid- or airfilled space separating the digestive tract from the outer bodywall. This bodycavity is also known as acoelom (from the Greek


koilos, hollow). A so-called "true" coelom forms from tissue derived from mesoderm. The inner and outer layers of tissue thatsurround the cavity connect dorsally and ventrally and formstructures that suspend the internal organs. Animals that possessa true coelom are known as coclomatcs (Figure n.aa).

Some triploblastic animals have a body cavity that isformed from mesoderm and endoderm (Figure 32,8b). Sucha cavity is called a "pseudocoelom" (from the Greek pseudo,false), and animals that have one are pseudocoelomates. Despite its name, however, a pseudocoelom is not false; it is afully functional body cavity. Finally, some triplobastic animalslack a body cavity altogether (Figure 32.8c). They are knowncollectively as acoelomates (from the Greek a, without).

A body cavity has many functions. Its fluid cushions the sus·pended organs, helping to prevent internal injury. In soft-bodiedcoelomates, such as earthworms, the coelom contains noncom-

pressible fluid that acts like a skeleton against which muscles canwork. The cavity also enables the internal organs to grow andmove independentlyofthe outer body wall. Ifit were not for yourcoelom, for example, every beat of your heart or ripple of yourintestine would warp your body's surface.

Current phylogenetic research suggests that true coelomsand pseudocoeloms have been gained or lost multiple times inthe course of animal evolution. Thus, the terms coelomatesand pseudocoelomates refer to grades, not clades.

Protostome and Deuterostome Development

Based on certain aspects of early development, many animalscan be categorized as having one oflv.'o developmental modes:protostome development or deuterostome development.These modes can generally be distinguished by differences incleavage, coelom formation, and fate of the blastopore.

CoelomCleavage

(a) Coelomate. Coelomates such as earthworms have a true coelom,a body cavity completely lined by tissue derived from mesoderm.

(cl Acoelomate. Acoelomates such as planarians lack a body cavitybetv\leen the digestive cavity and outer body wall,

Coelom Formation

A pattern in many animals with protostome development isspiral cleavage, in which the planes ofcell division are diagonalto the vertical axis ofthe embryo; as seen in the eight-cell stage,smaller cells lie in the grooves between larger, underlying cells(Figure 32,9a, left). Furthermore, the so-called determinatecleavageofsomeanimals with protostome development rigidlycasts ("determines~) the developmental fate ofeach embryoniccell very early. A cell isolated at the four-cell stage from a snail,for example, after repeated divisions will form an inviable embryo that lacks many parts.

In contrast to the spiral cleavage pattern, deuterostome development is predominantly characterized by radial cleavage. Thecleavage planesare either parallel or perpendicular to the verticalaxis of the embryo; as seen in the eight-cell stage, the tiers ofcellsare aligned, one directly above the other (see Figure 32.9a, right).Most animals with deuterostome development also haveindeterminate cleavage, meaning that each cell produced byearly cleavagedivisions retains the capacity to develop intoacorn·plete embryo. For example, if the cells of a sea urchin embryo areisolated at the four-cell stage, each will form acomplete larva. It isthe indeterminate cleavage ofthe human zygote t1lat makes identical twins possible. nlis characteristic also explains the developmental versatility ofembryonic stem cells that may provide newways to overcome a variety ofdiseases (see Chapter 20).

Another difference betv·:een protostome and deuterostome development is apparent later in the process. During gastrulation,an embryo's developing digestive tube initially forms as a blindpouch, the archenteron, which becomes the gut (Figure 32.9b).As the archenteron forms in protostome development, initiallysolid masses of mesoderm split and form the coelom. In contrast, in deuterostome development, the mesoderm buds fromthe wall ofthe archenteron, and its cavity becomes the coelom.

Muscle layer(frommesoderm)

Tissuefilled region(frommesoderm)

Body covering(from ectoderm)

Tissue layerlining coelomand suspendinginternal organs(from mesoderm)



Wall of digestive cavity(from endoderm)

.. Figure 32.8 Body cavities of triploblastic animals. Thevarious organ systems of a triploblastic animal develop from the threegerm layers that form in the embryo, Blue represents tissue derivedfrom ectoderm, red from mesoderm. and yellow from endoderm.

(b) Pseudo<.oelomate. Pseudocoelomates such as roundwormshave a body cavity lined in part by tissue derived from mesoderm,but also by tissue derived from endoderm.


.. Figure 32.9 A comparisonof protostome anddeuterostome development.These are useful generaldistinctions, though there are manyvariations and eKceptions to thesepatterns.

Protostome development(eKamples molluscs,

annelids)

Eight-cell stage Eight-cell stage(a) Cleavage. In general,

protostome developmentbegins with spiral. determinatecleavage. Oeuterostomedevelopment is characterizedby radial, indeterminatecleavage.

Spiral and determinate Radial and indeterminate

(b) Coelom formation. Coelomformation begins in thegastrula stage. In protostomedevelopment. the coelomforms from splits in themesoderm, In deuterostomedevelopment, the coelomforms from mesodermaloutpocketings of thearchenteron,Mesoderm

Folds of archenteronform coelom.

Blastopore!

Coelom

-.,--i""--,Archenteron -i---

Mesoderm

Solid masses of mesodermsplit and form coelom,

K.y• Ectoderm

• Mesoderm

Endoderm

Digestive tUbe---jff--

.......... MouthMouth develops from blastopore,

/Mouth

.......... AnusAnus develops from blastopore.

(c) Fate of the blastopore. Inprotostome development,the mouth forms from theblastopore. In deuterostomedevelopment. the mouthforms from a secondaryopening,

Fate of the Blastopore

1. Distinguish the terms grade and clade.2. Compare three aspects of the early development of a

snail (a mollusc) and a human (a chordate).3. _I,ll:f.i'IA Evaluate this claim: Ignoring the details of

their specific anatomy, worms, humans, and almost allother triploblasts are basically shaped like a doughnut.

For suggested answers, see AppendiK A.

Protostome and deuterostome development often differ in thefate ofthe blastopore, the indentation that during gastrulationleads to the formation of the archenteron (Figure 32.9c). After the archenteron develops, in most animals a second open

ing forms at the opposite end of the gastrula. Ultimately, theblastopore and this second opening become the two openingsof the digestive tube (the mouth and the anus). In protostomedevelopment, the mouth generally develops from the firstopening, the blastopore, and it is for this characteristic that theterm protostome derives (from the Greek protos, first, andstoma, mouth). In deuterostome (from the Greek deuteros, second) development, the mouth is derived from the secondaryopening, and the blastopore usually forms the anus.

CONCEPT CHECK 32.3

r~::'~;~2~~nimalphylogenyare emerging from moleculardata

Zoologists currently recognize about three dozen animalphyla. But the relationships between these phyla continue to bedebated. Although it might be frustrating that the phylogeniesin textbooks cannot be memorized as set-in-stone truths, theuncertainty inherent in these diagrams is a healthy reminder

that science is an ongoing, dynamic process of inquiry.Researchers have long based their hypotheses about animal

phylogeny on morphological data. In the late 1980s, biologistsalso began to study the molecular systematics of animals. Additional clues about animal phylogeny have come from newstudies of lesser-known phyla, along with fossil analyses thatcan help clarify which traits are ancestral in various animalgroups and which are derived.

As you read in Chapter 26, phylogenetic systematists seekto classify organisms based on common descent. Their goal isto place organisms into groups that correspond to clades,each of which includes an ancestral species and all of its


Echinodermata""..

Chordat;.;tt::t:

Brachiopoda

Ctenophora~

Ectoprocta '~

Cnidaria

Platyhelminthes

'o"'''iMollusca @Annelida~

ArthruPOd'~

Nematoda~

;;3••

-0•c

"'-- ...-•;;3 ,....-••

•• '--~~

~••~

0-I..~-

mC

3•~

2

•ANCESTRAL _ ~COLONIAL gFLAGELLATE AI

... Figure 32.10 A view of animalphylogeny based mainly onmorphological and developmentalcomparisons. The bilatenans are dividedinto deuterostomes and protostomes_ Agroup offlatworms known as Acoela (see Figure 32,11) is notshown in this tree because it was traditionally considereda subgroup of Platyhelminthes,n VVhich phylum is the sister group of Bilareria in this.. tree? Is the sister phylum the same in Figure 32.11?Points of Agreement

descendants. Based on cladistic methods, a phylogenetic tree takes shape as ahierarchy of clades nested within largerclades-the finer branches and the thick

er branches of the tree, respectively.Clades are inferred from shared derived

characters that are unique to membersofthe clade and their common ancestor.For example, a clade might be inferredfrom key anatomical and embryologicalsimilarities that researchers concludeare homologous. In recent years, molecular data such as DNA sequenceshave provided another source of information for inferring common ancestryand defining clades. But whether thedata used are "traditional" morphological characters or "new" molecular se

quences or a combination of both, theoverall goal is the same-to reconstruct

the evolutionary history of life.To get a sense of the debates that oc

cur in animal systematics, we'll compare

a traditional view of animal phylogeny,based primarily on morphological data(figure 32.10), with a view based mainlyon molecular data (figure 32.11).

The morphological and molecular trees agree on a number ofmajor features of animal phylogeny. After reading each point,see how the statement is reflected in the phylogenetic trees inFigures 32.10 and 32.1l.

I. All animals share a common ancestor. Both treesindicate that the animal kingdom is monophyletic,representing a dade called Metazoa. In other words, ifwe could trace all extant and extinct animal lineagesback to their origin, they would converge on a commonancestor.

2. Sponges are basal animals. Among the extant taxa,sponges (formerly, the phylum Porifera) branch fromthe base of both animal trees. Some molecular analysessuggest that sponges are paraphyletic, falling into at leastthe two phyla shown in Figure 32.11.

3. Eumetazoa is a dade of animals with true tissues.All animals except for sponges and a few other groupsbelong to a clade of eumetazoans ("true animals"). True

tissues evolved in the common ancestor of living eumetazoans. Basal members of the eumetazoan clade in

clude the phyla Ctenophora (comb jellies) and Cnidaria.These basal eumetazoans are diploblastic and generallyfeature radial symmetry.

4. Most animal phyla belong to the dade Bilateria.Bilateral symmetry and the presence of three germ layers are shared derived characters that help to define theclade Bilateria. This clade contains the majority of animal phyla, and its members are known as biJatcrians.The Cambrian explosion was primarily a rapid diversification of the bilaterians.

5. Chordates and some other phyla belong to thedade Deuterostomia. The term deuterostome refersnot only to a mode of animal development, but also tothe members of a dade that includes vertebrates andother chordates. (Note, however, that the traditional and

molecular views of animal phylogeny disagree as towhich other phyla are also deuterostomes.)

Progress in Resolving Silaterian Relationships

While evolutionary relationships inferred from traditionalmorphological data and molecular data are similar in manyrespects, there also are some differences. For example, themorphology-based tree in Figure 32.10 divides the bilaterians into two clades: deuterostomes and protostomes. Thisview assumes that these two modes of development reflect a


phylogenetic pattern. Within the protostomes, Figure 32.10 indicates that arthropods (which include insects and crustaceans)are grouped with annelids. Both groups have segmented bodies(think of the tail of a lobster, which is an arthropod, and anearthworm, which is an annelid).

A different view has emerged from molecular phylogeniesbased on ribosomal genes, Hox genes, and dozens of otherprotein·coding nuclear genes, as well as mitochondrial genes.

Collectively, these studies indicate that there are three majorclades of bilaterally symmetrical animals: Deuterostomia,Lophotrochowa, and Ecdysozoa (see Figure 32.11). In con

trast to the traditional morphological view, the molecularphylogeny holds that the arthropods and annelids are notclosely related to one another. Note also that Figure 32.11 includes a group of acoelomate flatworms (Acoela) not shownin Figure 32.10. Traditionally, acoel flatworms were classifiedwith other flatworms in the phylum Platyhelminthes. However, recent research indicates that acoel flatworms are basal

-

... Figure 32.12Ecdysis. Thismolting cicada is inthe process ofemerging from itsold exoskeleton.The animal Will

now secrete a new.larger exoskeleton.

bilaterians, not members of the phylum Platyhelminthes. Acoela's basalposition suggests that the bilateriansmay have descended from a commonancestor that resembled living acoelflatworms-that is, from an ancestor

that had a simple nervous system, asaclike gut, and no excretory system.

As seen in Figure 32.11, the molecular phylogeny assigns the animal phylathat are not in Deuterostomia to 1:\votaxa rather than one: the ecdysozoansand the lophotrochozoans. The cladename Ecdysozoa refers to a characteristic shared by nematodes, arthropods,and someofthe other ecdysozoan phylathat are not included in our survey.These animals secrete external skeletons (exoskeletons); the stiffcovering ofa cicada or cricket is an example. As theanimal grows, it molts, squirming outof its old exoskeleton and secreting alarger one. The process of sheddingthe old exoskeleton is called ecdysis(Figure 32.12). Though named for thischaracteristic, the clade was proposedbased mainly on molecular data thatsupport the common ancestry of itsmembers. Furthermore, some taxa ex

cluded from this clade by their molecular data, such as certain species ofleeches, do in fact molt

The name Lophotrochowa refers totwo different feahtres observed in someanimals belonging to this clade. Some

lophotrochozoans, such as brachiopods, develop a structurecalled a lophophore (from the Greek tophos, crest, andpflereiu, to carry), a crown of ciliated tentacles that function in

S;Ii'''~Calcarea~

Ctenophora r;:3

Acoela

Cnidaria

MOllllSca@

Annelida~

Nematoda f©ArthCOPPd;~

Brachiopoda

EChinodermata'"

Chorda'aA

m

Ie•

o•

Ie3••

••.-;,.•

m<3.. ~•2

r-----~----;;r----.---•

ANCESTRAL _ SCOLONIAL ~

FLAGELLATE a>

.. Figure 32.11 A view of animal phylogenybased mainly on molecular data. The bilateriansare divided into three main lineages: dellterostomes.lophotrochozoans. and ecdysozoans,


Future Directions in Animal Systematics

... Figure 32.13 Morphological characteristics foundamong lophotrochozoans.

32.4CONCEPT CHECK

think that the tree shown in Figure 32.11 is more strongly

supported than is the tree shown in Figure 32.10. Of course,

as new information emerges, our understanding of the evo

lutionary relationships shown in these trees may change.

Researchers continue to conduct large-scale analyses of mul

tiple genes across a wide sample ofanimal phyla. Abetter un

derstanding of the relationships bern'een these phyla will

give scientists a clearer picture of how the diversity of animal

body plans arose. In Chapters 33 and 34, we will take a closer

look at the diverse phyla of extant animals and their evolu

tionary history.

1. Describe the evidence that cnidarians share a mOTe

recent common ancestor with otheT animals than

with sponges.

2. How do the phylogenetic hypotheses presented in

Figures 32.10 and 32.11 differ in structuring the major

branches within the clade Bilateria?

3. N'WU'I. If Figure 32.11 accurately reflects phy

logeny, could both of the following be correct? (a) A

newly discovered arthropod fossil is 560 million years

old. (b) The most recent common ancestor of

Deuterostomia lived 535 million years ago. Explain.

For suggested answers, see Appendix A.

Apical tuitof cilia

'I/f,'!"'----Anus

(b) Structure of a trochophorelarva

(a) An ectoproct which has alophophore (LM)

Like any area of scientific inquiry, animal systematics is a

work in progress. At present, most (but not all) systematists

feeding (figure 32.13a}.lndividuals in other phyla, including

molluscs and annelids, go through a distinctive developmental

stage called the trochophore larva (Figure 32.13b)-hence

the name lophotrochozoan.

-&I4.if.- Go 10 the Study Area al www.masteringbio.comforBioFlix3-D Animations, MP3 Tutors, Videos, Practice Tests, an eBook, and more.

SUMMARY OF KEY CONCEPTS

_i,lili"_ 32.1Animals are multicellular, heterotrophic eukaryoteswith tissues that develop from embryonic layers(pp. 654-656).. Nutritional Mode Animals are heterotrophs that ingest

their food.

.. Cell Structure and Specialization Animals are multicellular eukaryotes. Their cells lack cell walls; instead, they are heldtogether by structural proteins such as collagen. Nervous tissue and muscle tissue are unique to animals.

.. Reproduction and Development In most animals, gastrulation follows the formation of the blastula and leads tothe formation of embryonic tissue layers. All animals, andonly animals, have Hox genes that regulate the development of body form. Although Hox genes have been highlyconserved, they can produce a wide diversity of animalmorphology.

-i iiiii'- 32.2The history of animals spans more than half a billionyears (pp. 656-658)... Neoproterozoic Era (1 Billion-542 Million Years Ago)

Early animal fossils include the Ediacaran fauna .

... Paleozoic Era (542-251 Million Years Ago) TheCambrian explOSion marks the earliest fossil appearance of manymajor groups of living animals.

.. Mesozoic Era (251-65.5 Million Years Ago) Dinosaurswere the dominant land vertebrates. Coral reefs emerged, providing marine habitats for other organisms.

.. Cenozoic Era (65.5 Million Years Ago to the Present)Mammalian orders diversified during the Cenozoic.

-i iiiii'_ 32.3Animals can be characterized by "body plans"(pp. 658-661).. Symmetry Animals may lack symmetry or may have radial or

bilateral symmetry. Bilaterally symmetrical animals have dorsal and ventral Sides, as well as anterior and posterior ends.

.. Tissues Eumetazoan embryos may be diploblastic (havingtwo germ layers) or triploblastic (having three germ layers).


••.•llil'_ 32.4New views of animal phylogeny are emerging frommolecular data (pp. 661-664)... Points of Agreement

... Progress in Resolving Bilaterian Relationships

Common ancestorof all animals

4. Acoelomates are characterized by

a. the absence of a brain.

b. the absence of mesoderm.c. deuterostome development.

d. a coelom that is not completely lined with mesoderm.

e. a solid body without a cavity surrounding internal organs.

5. Which of the following was probably the least important factor

in bringing about the Cambrian explosion?

a. the emergence of predator-prey relationships among

animals

b. the accumulation of diverse adaptations, such as shells

and different modes oflocomotion

c. the movement of animals onto land

d. the evolution of Hox genes that controlled development

e. the accumulation of sufficient atmospheric oxygen to

support the more active metabolism of mobile animals

6. What is the main basis for placing the arthropods and nema

todes in Ecdysozoa in one hypothesis of animal phylogeny?

a. Animals in both groups are segmented.

b. Animals in both groups undergo ecdysis.

c. They both have radial, determinate cleavage, and their

embryonic development is similar.

d. Fossils reveal a common ancestor of the two phyla.

e. Analysis of genes shows that their sequences are quite

similar, and these sequences differ from those of the

lophotrochozoans and deuterostomes.

EVOLUTION CONNECTION

-MH',. Vi,lt the Study Area at www.masteringbio.com for aPractice Test

For Self·Quiz answers, see Appendix A.

mo3

~o•

,.o•

Lophotroch020<l ~j.•g

Cnidaria

Acoela (basalbilaterians)

Deuterostomia

Sponges(basal animals)

Ctenophora

Truetissues

... Body Cavities In triploblastic animals, a body cavity may bepresent or absent. A body cavity can be a pseudocoelom (derived from both mesoderm and endoderm) or a true coelom(derived only from mesoderm).

.. Protostome and Deuterostome Development These twomodes of development often differ in patterns of cleavage,coelom formation, and fate of the blastopore.

Bilateralsymmetry

Three germlayers Ecdyso2oa

... Future Directions in Animal Systematics Phylogeneticstudies based on larger databases may provide further insightsinto animal evolutionary history.

Actl\'lty Animal Phylogfnetic Tree

Investigation How D<:> Molfcular Data Fit Traditional Phylogenies?

TESTING YOUR KNOWLEDGE

SELF-QUIZ

I. Among the characteristics unique to animals is

a. gastrulation. d. flagellated sperm.

b. multicellularity. e. heterotrophic nutrition.

c. sexual reproduction.

7. Some scientists suggest that the phrase "Cambrian fizzle" might

be more appropriate than "Cambrian explosion" to describe the

diversification ofanimals during that geologic period. Other sci·

entists have compared observing an explosion ofanimal diversity

in Cambrian strata to monitoring Earth from a satellite and

noticing the emergence of cities only when they are large enough

to be visible from that distance. What do these views imply

about the evolutionary history of animals during that time?

SCIENTIFIC INQUIRY

Cleavage Pattern Phyla

8. ••1&\1,1". Redraw the eumetazoan portion of Figure 32.11.

Using the information in the table below, label each branch that

leads to a phylum with 5, R, or I, depending on the cleavage

pattern of its members. \Vhat is the ancestral cleavage pattern?

How many times have cleavage patterns changed over the

course of evolution? Explain.

2. The distinction between sponges and other animal phyla is

based mainly on the absence versus the presence of

a. a body cavity. d. true tissues.

b. a complete digestive tract. e. mesoderm.

c. a circulatory system.

3. Which of these is a point ofconflict between the phylogenetic

analyses presented in Figures 32.10 and 32.11?

a. the monophyly of the animal kingdom

b. the relationship of taxa of segmented animals to taxa of

nonsegmented animals

c. that sponges are basal animals

d. that chordates are deuterostomes

e. the monophyly of the bilaterians

Spiral (S)

Idiosyncratic (I)

Radial {R)

Mollusca, Platyhelminthes, Annelida

Acocla, Arthropoda

All eumetazoan phyla not listed above


r:~~~j~;a~~~ulticellular,rcastilho.pt/da/calendario_files/cap_32_campbell.pdfthe resulting...

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