The Evolution of Animal Diversity

TheEvolution of Animal Diversity

Objectives Introduction Describethe difficultiesof classifyingthe duck-billedplatypus and otherAustralian mammals. AnimalEvolution and Diversity 18.1 Define animalsand distinguishthem from other forms of life. 18.1 Describethe generalanimal life cycle and the basic body plan. 18.2 Describethe five-stagehypothesis for the evolutionof animalsfrom protists' 18.2 Describethe Cambrianexplosion and list threehypotheses to explainits occurrence. lnvertebrates 1E.!18.15 Describethe characteristicsof anddistinguish between the following phyla: Porifera, Cnidaria,Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata, Chordata. 1831E.15 Distinguishbetween radially and bilaterallysymmetric animals. Note which body type is found in each of the phyla examinedin this chapter. 18.7,1E.8 Distinguishbetween a pseudocoelomand a coelom.Describe the functionsof eachand note the animal phyla where they occur. 18.10 Define segmentation,explain its functions, and note the animal phyla where it occurs. 18.13 Describethe commoncharacteristics of insects.Distinguish between the seveninsect orders describedin this chaPter. Vertebrates 18.16 Describethe definingcharacteristics of vertebrates. 18.17-1E.22 Describethe characteristicsof and distinguishbetween the following vertebrategroups: agnathans,Chon&ichthyes, Osteichthyes, Amphibia, Reptilia,Aves, Mammalia.

Phylogenyof the Animal Kingdom 18.23 Describethe fwo main classificationsof the major animal groups.Explain why there are differencesin thesetwo systems. 18.24 Describethe consequencesof introducedspecies in Australia.

KeyTerms kingdomAnimalia mesoderm radial symmetry ingestion larva choanocyte blastula metamorphosis amoebocyte gastrula invertebrate suspensionfeeder ectoderm sponge choanoflagellate endoderm Porifera Cnidaria t73 r74 Instructor's Guide to Textand Media

polyp segmentatron lancelet medusa Annelida skull gasfrovascularcavity earthworm backbone cnidocyte polychaete vertebra bilateral syulmetry leech agnathan anterior Arthropoda Chondrichthyes posterior arthropod Osteichthyes dorsal exoskeleton cartilaginousfish ventral molting lateral line system lateral horseshoe crab bony fish flatworm arachnid operculum Platyhelminthes crustacean swim bladder free-living flatworm millipede ray-finnedfish fluke centipede lobe-finnedflsh tapeworm entomology lungfish bodycavity incomplete metamorphosis Amphibia pseudocoelom complete metamorphosii Reptilia coelom echinoderm amniotic egg roundworm Echinodermata ectothermic Nematoda endoskeleton endothermic mollusk water vascular system Aves Mollusca Chordata Mammalia foot dorsal, hollow nerve cord monotreme visceralmass notochord placenta mantle pharyngeal slit marsupial radula post-anal tail placental gastropod chordate eutherian bivalve vertebrate protostome cephalopod tunicate deuterostome

Word Roots choano- : a funnel; -cyte : cell (choanocyte:flagellated collar cells of a sponge) cnido- : a nettle (cnidocytes:unique cells that function in defenseand prey capturein cnidarians) cuti- : the skin (cuticle: the exoskeletonof an arthropod) deutero- : second(deuterostome.' one of two distinct evolutionarylines of coelomatescharacterized by radial, indeterminatecleavage, enterocoelous formation of the coelom, and developmentof the anusfrom the blastopore) echino- : spiny;-derm : skin (echinoderm.'sessile or slow-movinganimals with a thin skin that cov- ers an exoskeleton;the group includessea stars, sea urchins, brittle stars,crinoids, and basketstars) ecto- : outside; -derm : skin (ectoderm:the outermostof the three primary germ layers in animal embryos) endo- : inner; themr- : heat (endotherm.'an animal that uses metabolicenergy to maintain a constant body temperature,such as a bird or mammal) gastro- : stomach;-vascula : a little vessel(gastrovascular cavity: the centraldigestive compartment, usuallywith a singleopening that functionsas both mouth and anus) in- : without (irwenebrate:an animal without a backbone) Chapter 18 TheEvolution of Animal Diversity t75

: l marsupi- a bag, pouch (marsupial:a mammal,such as a koala,kangaroo, or opossum,whose young completetheir embryonicdevelopment inside a maternalpouch called the marsupium) meso- : middle (rnesoderm:the middle primary germ layer of an early embryo) meta- : boundary,tuming point; -morph : form (metarnorphosis;the resurgenceof developmentin an animal larva that transforms it into a sexually mature adult) mono- : one (rnonotreme;an egg-layingmammal, represented by the platypusand echidna) noto- : the back; -chord : a string (notochord:a longitudinal,flexible rod formed from dorsalmeso- derm and locatedbetween the gut and the nervecord in all chordateembryos) proto- : first; -stoma : mouth Qtrotostome.'a memberof one of two distinctevolutionary lines of coelomatescharactenzed by spiral,determinate cleavage, schizocoelous formation of the coelom,and developmentof the mouth from the blastopore)

LectureOutline Intrcduction WhatAm I? A. Most known organismsare animals. 1. Of the 1.5 million known species,two-thirds are animals. Animal diversity progressedthrough millions of yearsof evolutionand naturalselection. 2. Humanshave a tendencyto appreciateand studyanimals, but what is an animal? Do we know one when we seeone and how do we classifydifferent animals? B. What am I (Figure18.0)? 1. The duck-billedplatypus was a taxonomicnighnnare when initially found by the first Europeanswho visited Ausftalia. It lays eggs,has a duck bill and webbedfeet . . . it's a bird-no, it has fur, a tail like a beaver,and mammary glands . . . it's a mammal- well . . . a specialmammal called a monofteme(an eggJayingmammal)! 2. Australia is full of specialmammals called monotremes and marsupials.The niches that were being filled with placentalmammals on other continentswere left empty when Australia broke away from Pangaea.Without competitionfrom other placental mammals,monotremes and marsupialsfilled in the empty niches and becamethe dominantmammals in Australia. Preview:Zoologists distinguish between animals with internalskeletons (vertebrates)and those without internal skeletons(invertebrates). All but one animal phylum (our own, phylum Chordata)are invertebrates(Module 18.15).

I. Animal Evolution and DiversitY Module 1E.1 What is an animal? A. Animals are eukaryotic,multicellular heterotrophsthat lack cell walls and obtain nutri- tion by ingestion (Figure 18.1A).This distinguishesanimals from fungi that digesttheir food first and then absorbit. Most animals havemuscle cells for movementand nerve cells for sensoryperception. B. The life cycle of most animalsincludes a dominant,diploid adult that produceseggs or spermby meiosis.These gametes fuse to form a zygote.The zygotedevelops into the adult animal, passingthrough a seriesof embryonicstages, many of which are shared by most membersof the animalkingdom (Figure18.1). Review:This is an exampleof homology(Module 15.11). 176 Instructor'sGuide to Textand Media

C. In all animalsthe embryonicstages include the blastula (hollow ball of cells) and, in most,a gastrula(a saclikeembryo with one openingand two layers of cells). Most further developan additionallayer of cells. Preview:Embryonic development (Modules 27.9-27.I5). D. In many animals,the gastruladevelops into one or more immaturestages, for example, larvae that developinto the sexually mature adults only after metamorphosis. NOTE: Larvaeare dispersivestages that help an animal'soffspring find suitablehabitats before continuingto grow. They are also extremely important sourcesof food for many other animalsin aquatichabitats. E. Animals haveunique types of cell junctions(Module 4.19). F. Animals are,with the exceptionof the gametes,composed of diploid cells. G. Specialregulatory genes called Hox genesconftol zygotedevelopment. These homeotic genesare presentonly in animals.

Module L8.2 The animalkingdom probably originated from colonial protists. A. Fossilsof the oldestknown animalsdate from the late Precambrianera, =600 mya. B. A hypotheticalevolutionary scenario leading to the first animalproceeds as follows: 1. Colonialprotists of a few, identical,flagellated cells 2. coloniesthat ingested organic materials suspended in the li::';::ti:-;:rl;f;:1 3. Colonieswith cells ,p""iuti""a for somatic(movement, digestion, etc.) and repro- ductive functions 4. Differentiatedentities with an infolded, temporary digestiveregion 5. "Protoanimals,"completely infolded, with two-layeredbody walls (Figure 18.2) C. Theseprotoanimals probably "crawled," feeding on the oceanbottom. D. The modernanimal phyla evolved during the Cambrianperiod, =545 mya, a ten-million-year period of explosiveevolution. E. Three hypothesesexplain the rapid expansionof animal diversity during the early Cam- brian era: L Ecologicalcauses; increased dependency on the predator/preyrelationship. 2. Geologicchanges; atmospheric oxygen may have reacheda critical threshold that was high enoughto supportthe active lifestyle of animals. 3. Geneticcauses; the developmentof the Hox genessupported the diversity associ- ated with variationsin the spatial and temporal arrangementof the genesduring embryonicstages of development. NOTE: Somemutualistic synthesis of the three hypothesesis likely the correct answer.

II. Invertebrates Module 18.3 Spongeshave relatively simple, porous bodies. A. Spongesare classifiedin the phylum Porifera. Most are marine and live singly, attachedto a substrate,and rangein height from I cm to 2 m (Figure 18.34). B. Many spongesare built on a body plan havingradial s5rmmetry,that is, similar shapes as mirror imagesaround a centralaxis (Figure 18.38). C. The body of a spongeconsists of two layersof cells. An inner layer of flagellatedcells, called choanocytes,sunound an inner chamberand sweepthe water toward the larger Chapter 18 The Evolution of Animal Diversity

pore; amoebocytesproduce the skeletal fibers or proteins called spongin (Figure 18.3C).More complexsponges have folded body walls and branchedwater channels (Figure18.3D). D. Spongesfeed when the choanocyteflagella beat, creating a flow of waterin throughthe pores,through the collars,trapping bacteria in mucus,and out throughthe large,upper opening.The choanocytesthen phagocytize the food and packageit in vacuoles.Amoe- bocytespick up the food vacuoles,digest the food, and carry the nutrients to other cells. Preview:Conftast this with the structureof the digestive system of other animals (Module21.3). E. In additionto digestingand distributingfood, amoebocytesalso transportoxygen, dis- poseof waste,and manufactureskeletal elements.Further, amoebocytescan changeinto othercell types. NOTE: Spongesfunction as complex coloniesof differentiated but interchangeablecells, the amoebocytesbeing the central cell type that forms most of the others.If a spongeis pressedthrough a sieve and all the cells and skeletal elements are separated,they will reorganizethemselves back into the samelayers and similar shape. F. Spongesare likely to have beena very early offshoot from the multicellular organisms that gaverise to the animals (Figure 18.23).Sponges retain severalprotistan characteris- tics, including not having a digestivefract and having intracellular digestion.Develop- mentally,sponges do not go through a gastrulastage. Sponge cells do not maketissues as in other animalsand spongeslack nervesand muscles although individual cells can senseand react to changesin their surroundings. G. Spongeslikely descendedfrom choanoflagellates(Figure 18.38) in the late Precam- brian. Choanoflagellatesare colonial protists composedentirely of collar cells (choanocytes)but, as protists, they do not show cellular differentiation.

Modute 18.4 Cnidariansare radial animalswith stinging threads. A. Cnidarians exhibit radial symmetry. B. Theseanimals may be in the form of a polyp (relatively fixed in position) or a medusa(swimming), or they may alternatebetween polyp and medusaforms. Both body plans have a central tubular body (sunounding a gastrovascular cavity), one opening("mouth") into this cavity, and tentaclesarranged around the mouth (Figure 18.4A,B, C). C. Along the tentaclesare cnidocytes (stinger cells) that function in defenseand the cap- turingof food. The coiled threadin eachcell is discharged,stinging or entanglingthe preyor predatoras it brushesagainst the cnidocyte(Figure 18.4D). D. Cnidariansfiap food with their tentacles,then maneuverit into a gastrovascularcavity whereit is digestedand distributedthroughout the body. Undigested food is eliminated throughthe mouth. E. Cnidariansare built at the tissuelevel of construction,with severaldifferent cell types anangedin layers and having cornmonfunctions. Muscle cells are arrangedin groups that allow the body to extend,move tentacles,and contract.Nerve tissuecoordinates this movement.But unlike the rest of the animal phyla (except sponges),cnidarians only havetwo types of embryonictissue, an outer epidermis and an inner gastrovascular cell layer, no mesodenn. F. Developmentincludes a gastrulastage, like all the remaining animal phyla. 178 Instructor's Guide to Tbxt and Media

Module 18.5 Most animals are bilaterally symmetrical. Bilateral symmetry means that an animal can be divided into minor images (left and right sides)by a single plane (Figure 18.5). B. Associatedwith bilateral symmetryis the division of the animalinto head(anterior)' tail (posterior), back (dorsal), bottom (ventral), and side(lateral) surfaces. C. The headhouses sensory structures and the brain. D. Bilateral animals are fundamentalty different from radial animals.In contrastto radial animalsthat sit or drift passively,bilateral animals actively move through their environments headfirst.Bilateral symmetry,a head,and forward movementare important evolutionary developments.

Module 18.6 Flatwormsare the simplestbilateral animals. A. Flatwor'ms are classified in the phylum Platyhelminthes that is composedof approxi- mately 20,000species. There are threemajor groupsof flatworms:free'living planarians that live on rocks in marine and fresh water, parasiticflukes, and tapeworms. B. Like cnidarians,planarians and most flukes have a gasftovascularcavity and no other body cavities. The body normally has a headend with a concentrationof sensory nerves.The mouth opens from the ventral surfaceand the gastrovascularcavity branchesthrough the entire length of the body (Figure 18.64). NOTE: Flatworms are not quite at the organ level of construction. c. Most flukes have a complex life cycle, including reproductionin more than one host and one or more larval stages.Schistosoma (blood fluke) adultslive and permanently mate inside blood vessels,where they feed on blood.As manyas 1000fertilized eggsa day are produced,and they leave through the host's intestine.The eggsgrow into larvae that infect snails. Asexual reproductionin the snail producesdifferent larvae that infect humans(Figure 18.68). D. Thpeworms are highly adaptedparasites that inhabit the digestivetracts of their hosts. Unlike other flatworms, tapewormsare segmentedand lack a gastrovascularcavity, absorbingtheir predigestedfood directly. The anterior end (the head)attaches to the host with hooks and suckers,and a region behind generatessegments. Sexual reproduction occurs in each segment;the oldest segmentsbreak off and leavethe host via the feces. Many tapewormsproduce larvae that infect the prey animal,while the adult tape- wonns infect that prey animal'spredator (Figure 18.6C).

Module 18.7 Most animals have a body cavity. A. Review:Animal development(Figure 18.1). B. Depending on how tissue regions developfrom the gastrulastage, three body plans exist among animal phyla: 1. The body is solid, except for the gasftovascularcavity, as in flatwormsand cnidaria (Figures18.7A). Z. The body contains a pseudocoelom,an internal spacein direct contactwith the inner layer of the digestivetract, as in roundworms(Figures 18.78). 3. The body contains a coelom, an internal spacecompletely lined by tissue of the middle layer, as in all other animals.(Figures 18.7C). c. For each body plan, the skin and other outer layers developfrom the outside of the gastrula, the inner lining of the digestivefract or gastrovascularcavity (yellow) develops irom the inside of the gastrula, and the middle layer (pink) developsfrom outgrowths of the inner layer. Chapter18 TheEvolution of Animal Diversity 179

D. Previe'w.'TWo differentpaths of developmentof the middlelayer distinguishtwo lines of animal evolution (Figure 18.23). E. Advantagesof having a body cavity includehaving greater flexibility, cushioningof internal organs, and providing room for internal organ growth. F. In someanimals (like earthworms),body cavitiesunder pressure function as a skeleton. G. The fluid in the cavity circulatesnutrients and oxygenand aids in wastecollection and disposal.

Module 18.E Roundwonnshave a pseudocoelomand a completedigestive tract. A. Roundworms, classifiedin the phylumNematoda, are numerous and diversein most environments.Most are importantdecomposers or parasitesof plantsor animals. B. The roundworm body is cylindrical, includesa completeintestinal tract, and is covered by a tough, nonliving cuticle (Figure 18.8A). C. Food passesin one direction along a digestivetract that includesregions specializedfor certain functions: food intake, breakup,digestion, and absorption,and wasteelimination. NOTE: Roundworms are tending toward the organ level of construction. D. One free-living species,Caenorhabditis elegans, is an importantorganism for genetic researchand one of the best-understoodorganisms. Researchers have been able to trace the developmentallineage of eachof an adult's 1000cells. E. Trichinosisis a diseasecaused by the roundwormTrichinella spiralis. Humansget this parasiteby eating raw pork (Figure 18.88).

Module 18.9 Diverse mollusks are variationson a commonbody plan. A. Mollusks are classifiedin the phylum Mollusca. B. The basic body plan of a mollusk includesbilateral symmetry,a completedigestive tract, a coelom, and many internal organs. C. Two distinctive characteristicsof the phylum are a muscular'toot" and a mantle, an outgrowth of the body surfacethat drapesover the animal,functions in sensoryrecep- tion, often secretesa shell, and usuallyhouses gills that functionin gas exchangeand waste removal. D. Mollusks also have a true circulatory system(in conftastto the circulatory function of the gastrovascularcavity of cnidariansand flatworms,and the pseudocoelomof roundworms) and separatesex organs.The life cycle includesa ciliated larva called a tro- chophore, a trait usedin taxonomy. E. Most have a radula, an organ usedto scrapefood, suchas algae, off surfacesin the environment(Figure 18.9A). F. Evolution has modified the basic body plan in different groupsof mollusks. Gastropods include snails and slugs.Bivalves include clams,scallops, and oysters. Cephalopodsinclude octopusesand squids(Figure 18.9B-F). G. Cephalopods are built for speed,and have large brains and sophisticatedsense organs, especiallytheir eyes,which can focusa clearimage on the retina.

Module 1E.10 Many animalshave a segmentedbody. A. Segmentation is the subdivisionof the body into repeatedparts B. The segmentsin an earthwormare clearly visible from the outside,outlining the repeating pattern of organsinside. The nervous,circulatory and excretory systemsall have repeating,mostly identical,parts in eachsegment (Figure 18.10,4,). 180 Instractor's Guide to Tbxt and Media

C. ln other animals, segmentationmay involve fewer segmentsor be less obvious (Figure 18.108,C). D. Advantagesof segmentationinclude greater body flexibility and mobility. For example, in the earthworm, rhythmic alternating contractions and elongations of segments propel the worm into or along the ground. In many animals, the segmentsare the sites of insertion of walking legs or muscles. NOTE: There are two important evolutionary advantages. Genetically speaking, it is easierto build a large,complex animal by repeatinga single developmentalsequence in many smallerunits thanby following a longersequence of developmentalsteps for the whole region.Also, duringevolution, animal groups(polychaete annelids, the insects and other arthropods,and the chordates)have segmentsspecialized into different func- tional regions,working from a basicpattern; segments are often fused.But how did segmentationevolve? This is a questionthat is currently being debatedamong evolu- tionists and the answermay be found in the field of evo-devo.

Module 18.11 Earthwonnsand otherannelids are segmentedworms. A. Thesewonns are classifiedin the phylum Annelida. They live in the sea,in most fresh- water habitats,and in damp soil. Annelids usually have one or mdre anterior segments speciallymodified into a headregion. B. Earthworrns are one groupof annelids adaptedto life in soil. They consumethe soil, digest the organicparts, and eliminate undigestedsoil and other wasteproducts in their feces,improving soil texturein the process. C. Polychaeteworms are mostly marine inhabitants.They are characterizedby segmental appendageswith broad,paddlelike appendages and bristles. Somepolychaetes show modificationof anteriorsegments (Figure 18.11A,B). The appendagesalso increasethe surface-to-volumeratio and increase02 uptake and waste removal. D. Leechesare free-living carnivoresof aquatic animals or blood-suckingparasites on vertebrates(Figure 18.11C). A blood-suckingleech cuts the skin with razor-sharpjaws and secretesan anestheticand an anticoagulant.Leeches are still usedin medicine, and their anticoagulantsare being producedby genetic engineering.

Module 18.12 Arthropodsare the most numerousand widespreadof all animals. A. In terms of diversity,geographical distribution, and sheernumbers, the phylum Arthro- poda is the most successfulthat has ever lived. B. Basedon the presenceof Cambrianfossils that displaycharacteristics of both annelids and arthropods, it has beenthought that arthropodsevolved from annelids or segmentedancestors of annelids. C. However,molecular evidence indicates that annelidsand arthropodsindependently evolved from bilaterally symmetricaland segmentedanimals. D. Arthropodsare segmented(often fused), havejointed appendages,and have an exoskeletoncomposed of chitin and proteins(Figure 18.12A). Preview:To facilitatemovement, muscle tissue is attachedto the inside of the exoskeleton (Module 30.2). E. To gro*, arthropodsmolt their exoskeleton,swell in size, and secretea new, developing exoskeleton F. Horseshoecrabs are "living fossil" life forms that have survivedfor hundredsof millions of yearswith little change(see Module 14.8:equilibrium). A very close relativeof the moderngenus was abundant300 mya (Figure 18.128). Chapter 18 The Evolution of Animal Diversity 181

G. Arachnids includescorpions, spiders, ticks, and mites.Their ancestorswere amongthe first terrestrialcarnivores. Except for mites,arachnids are carnivores(Figure 18.12C). H. Crustaceansinclude crabs, shrimps, lobsters, crayfish, and barnacles;they are mostly aquatic(Figure 18. 12D). Mitlipedeshave segments with two pairs of appendageseach and feed on decaying plants.Centipedes have segments with one pair of appendageseach and are carnivorous.Both grcupsare telrestrial(Figure 18.128).

Module 18.13 Insectsare the most diversegroup of organisms. A. About onemillion insectspecies are known to biologists(entomologists are biologists who studyinsects), representing perhaps half of thosethat exist. Insectshave been importantaspects of tenestriallife for 400 million years,less so in aquaticand, espe- cially, marinehabitats. B. Insectsare united as a group in having a three-partbody plan: head, thorax, and abdomen.The headhas sensoryappendages and mouthpartsspecialized for a particular diet. The thoraxcontains three pairs of walking legs and,usually, one or two pairs of wings (Figure18.13A). The abdomenhouses digestive and reproductiveorgans. C. Metamorphosisis commonto many insects.Incomplete metamorphosis(Orthoptera, Odonata,and Hemiptera)results in an adult that resemblesthe young but is larger and has different body proportions.Complete metamorphosis (Coleoptera,Lepidoptera, Diptera, Hymenoptera)occurs when a larval form specializedfor eating and growing developsinto a very different adult form that is specializedfor reproduction and dispersal. D. Grasshoppers(order Orthoptera)have biting and chewing mouthparts.Most speciesare herbivorous(mantids are an exception).They have two pairs of wings. The forewing is thickenedand the hind wing is membranous. E. Damselfliesand dragonflies(order Odonata)have biting mouthpartsand are carnivorous. They havetwo identicalpairs of wings (Figure 18.138). True bugs (order Hemiptera)have piercing, sucking mouthparts,and most speciesfeed on plant sap(bedbugs feed on blood).They havetwo pairs of wings (Figure 18.13C). G. Beetlesmake up the largestorder (Coleoptera)in the animal kingdom, with some 500,000species known worldwide from all types of habitats.They have biting and chewingmouthparts, and are carnivorous,omnivorous, or herbivorous.They have nvo pairs of wings, and the forewings serveas protectivecovering for the hindwings (Figure l8.l3D). H. Moths and butterflies(order Lepidoptera)are the secondmost numerousinsects. They have drinking-tubemouthparts for sipping nectaror other liquids, and have two pairs of scale-coveredwings (Figure 18.13E). L Flies, gnats,and mosquitoes(order Diptera) have lapping mouthparts and feed on nectar or otherliquids (mosquitoeshave piercing, sucking mouthparts and suck blood).They havea singlepair of functionalwings, with the hindwingsreduced to halteresto maintain balance(Figure 18.13F). J. Ants, bees,and wasps (order Hymenoptera) are the third most numerousinsects. They havechewing and suckingmouthparts, and many are herbivorous.Theyhave two pairs of wings. Many in this group display complex behavior and social organization(Figure 18.13G). "superorganisms." NOTE:These social groups function as Someaspects of their social behaviorare covered in Chapter37. L82 Instructor'sGuide to Textand Media

Module 18.14 Echinodermshave spiny skin, an endoskeleton,and a water vascularsystem for movement. The phylum Echinodermata includessea stars, sand dollars, and seaurchins (all marine)and representsa secondbranch of evolutionin the animalkingdom. Similarities in embryonicdevelopment suggest that this phylum is closelyrelated to our own,the phylum Chordata. B. Echinoderms lack segmentationand bilateral symmetry as adults, but larvaeare bilaterally symmetrical.Members of this phylum are notedfor their regenerativecapacity. Most havetubular endoskeletons composed of fusedplates lying just underthe skin. Uniqueto this phylum is the presenceof a water vascular system.This is a network of water-filledcanals that branch into extensionscalled tube feet that functionin movement,ingestion, and gasexchange (Figure 18.144). "arms" c. Seastars have flexible that bear the tube feet. They wrap thesearms around a bivalve prey, pull the valves apart, extrude their stomachout their mouth and into the opening,and digestthe soft parts (Figure 18.148). D. Seaurchins are spherical,with five double rows of tube feet running radially, with which they pull themselvesalong. They eat algae(Figure 18.14C).

Module 1E.15Our own phylum, Chordata, is distinguishedby four features. A. A dorsal, hollow nerve cord. B. A notochord: a flexible, longitudinal rod locatedbetween the digestivetract and the nervecord. C. Pharyngeal slits, which are gill structures(slits and supports)in the pharynxregion behind the mouth. D. A muscularpost-anal tail. E. The most diversechordates are vertebrates.However, there are severalinvertebrate $oups in this phylum. F. Lancelets are small, bladelike chordatesthat live anchoredby their tails in marine sandsand exposetheir headsand mouths, filtering and trapping organic particlesin mucus aroundthe gill slits. These animals show the clearestpresentation of the chor- datebody plan (Figure 18.158).Molecular evidence indicates that lanceletsare the closestliving relativesof vertebrates. H. Itrnicates (seasquirts) are anothergroup of marinechordates (Figure 18.15A).As adults,they do not exhibit the chordatepattern of notochord and nerve cord. As stationary filter feeders,they use gill slits much like lanceletsdo. Larval tunicatesexhibit the completechordate pattern and look very much like adult lancelets.It is likely that during the Cambrian(500 mya), by paedomorphosis(Module 15.7),vertebrates evolved from chordatessimilar to larval tunicates.

III. Vertebrates Module 18.16 A skull and a backboneare hallmarksof vertebrates. A. A skutt forms a casefor the brain (Figure 18.16). B. A segmentedbackbone composed of vertebraeencloses the nervecord. C. Most vertebrateshave skeletal support for pairedappendages (fins, legs,arms, wings). D. The vertebrateskeleton is an endoskeletonof cartilage or bone. Thesenonliving materi- Chapter 18 The Evolution of Animal Diversity 183

als containliving cells and grow as the animal grows,unlike the exoskeletonsof arthropods,which must be molted prior to growth.

Module L8.17 Most vertebrateshave hinged jaws. A. One group of vertebrates,the lampreys(class Agnatha), lack jaws but haveskeletal supportsbetween their gill slits, and they lack pairedappendages. Otherwise, they are superficiallysimilar to fishes.Most are parasiteson fish, boring a holein the host and suckingits blood (Figure 18.17A). B. Jawsevolved by modificationof the first two pairs of skeletalsupports of the gill slits. Similar eventsoccur during the embryonic developmentof all fishestoday (Figure 18.178). C. The oldestfossils of jawed vertebratesappear in rocks that were formed450 mya.Jaws enabledvertebrates (mostly fish) to catch and consumea wider variety of foods than were availableto filter feeders,thus replacing them (the agnathans).

Module 1E.18Fishes are jawed vertebrateswith gills and pairedfins. A. Fish extract oxygen from water with their gills. Their paired fins help stabilizetheir bodies(Figure 18.18C). B. The cartilaginous fishes (classChondrichthyes) include the sharksand rays and have skeletonsof flexible cartilage.Sharks have a keen senseof smell andsharp vision, and can senseminute vibrationswith a pressure-sensitivelateral line system.Because they cannotpump water through the gills, they must swim in order to movethe water (Fig- ure 18.18A). C. Bony fishes (class Osteichthyes) are more common and diverseand havea stiff skeleton of bone reinforced by hard calcium salts. Like sharks,they havea keen senseof smell and a lateral line system.They also have a keen senseof sight, and a bony flap over the gills called an operculum that helps move water through the gills when the fish is stationary(Figure 18.188). D. Bony fish also have a swim bladder that can act as a buoyantcounter-balance to their heavierbones. Sometimes this is connectedto the digestivetract, enabling certain speciesto gulp air to increaseoxygen intake. E. Bony fish, the largest class of vertebrates,are divided into two groups,the ray-finned fish and the lobe-finned fish. Thin, flexible skeletalrays supportthe fins of ray-finned fishes.The fins of lobe-finnedfishes are muscularand supportedby bones.

Module fE.19 Amphibians were the first land vertebrates. A. Amphibians ins1.6t frogs, toads,and salamanders. B. Most amphibiansare tied to water becausetheir eggs and larvae (tadpoles)develop in water. C. Tiadpolesare aquatic, legless scavengerswith gills, a tail, and a lateralline system(Fig- ure l8.l9A). D. They undergoa radical metamorphosis(Figure 18.198)to changeinto an adult that is often a terrestrial hunter, with paired legs, externaleardrums, air-breathing lungs, and no lateralline (Figure 18.19C). E. The documenteddecline of the amphibianpopulation worldwide over the last 25 years has zoologistsconcerned. The decline may be attributableto acid rains that harm the eggsand larvaeof amphibians(review Module 2.16). LU Instructor's Guide to Text and Media

Amphibians were the first land vertebratesand evolved about 400 mya from either lobed-finnedf,shes or lungfishes.Structurally, ancestral lobe-finned fish had shong fins, which may havebeen used to paddleand wriggle over densevegetation, and saclike lungs (and gills). Molecularevidence indicates that ancestrallungfishes, which also had saclikelungs, gave rise to the first land vertebrates.A good exampleof a transition animal (from fish to amphibian)is the fossil recordof Acanthostega,a four-leggedfish from the Devonianera (Figure18.19D). G. Early amphibiansthrived on insectsand otherinvertebrates in the coal-producingforests of the Carboniferousperiod (Module17.7). This periodis known as the Age of Amphibians.

Module 18.20 Reptiles have more terrestrialadaptations than amphibians. A. Reptiles include snakes,lizards, turtles, alligators, and crocodilians.Lizards are the most numerousand diversereptile group, crocodilesand alligators are the largest, snakeslikely descendedfrom lizards,and turtleshave changed little sincethey evolved. Review:Reptiles are not a monophyleticgoup. Cladisticanalysis groups birds with crocodilesand lizards with snakes,and places the turtlesin anotherclade (Module 1s.r3). B. Reptiles have skin protectedwith protein (keratin), eggswith coatingsthat retain water, an internal fluid-filled sac (amnion)that bathesthe embryo, and a food supply (yolk) in the amniotic egg. The young hatchas juveniles, bypassingthe need for free-living larvae (Figure 18.204). c. Most modernreptiles are ectothermic.They wann up by absorbingexternal heat, rather than generatingmuch of their own metabolicheat as endothermic animals do (Figure 18.208). D. Following the decline of prehistoricamphibians, reptitan lineagesexpanded and domi- nated Earth during the Age of Reptiles(20M5 mya). Thesedinosaurs may have been endothermic(Figure I 8.20C). E. When the dinosaursdied off =65 mya (Module 15.5),one line survivedand evolved into birds.

Module 18.21 Birds sharemany featureswith their reptilian ancestors. Birds (classAves) have amniotic eggs,scales on their legs, and a reptilian body form. They evolvedfrom one line of dinosaurs150-200 mya. B. Feathers,the most distinctive characteristicof birds, are derived from scales.Feathers shapebird wings into airfoils that createlift and enablebirds to maneuverin the air. c. Archneopteryxis an extinct bird, with feathersand characteristicsof bipedal dinosaurs such as teeth,claws, and a tail with manyvertebrae (Figure 18.214). With its heavy body,Archaeopteryx is morelikely to havebeen a glider than a flier Archaeopteryxis not the ancestorof modernbirds. D. Many bird groups becameextinct about65 mya, along with the rest of the dinosaurs. Thus, the bird lineageappears to havegone through a bottleneck(Module 13.11)about 65 mya (the 65-mya massextinction) and modernbirds evolved from a very few sur- viving groups. E. Modern birds have additional adaptationsfor flight, including an absenceof teeth, no claws on their wings, very shorttailbones, hollow bones,large breast muscles, efficient lungs, and an extremely high rate of metabotsm to provide energy.Birds also have relativelylarge brains and possiblythe bestvision of all vertebrates(Figure 18.218). Birds are fabulousfliers, thoughsome birds are flightless(Figure l8.2lc). Chnpter18 TheEvolution of AnimalDiversity 185

Module 18.22 Mammals also evolvedfrom reptiles. A. Mammals evolvedfrom very early reptiles,about 225 mya,but occupiedminor parts of habitats during the Age of Reptiles.Once the dinosaursdied off, the mammalian lineagesunderwent adaptive radiation. Review:Adaptive radiation(Module 14.4). B. Most mammalsare terrestrial,with a numberof wingedand totally aquaticspecies. The largestanimal that has ever existedis the blue whale,a speciesthat can reach30 m in length. C. Mammalsare endothermic,and they havehair and mammaryglands. D. The monotremes,such as the platypus,are egg-layingmammals that live in Australia and Tasmania(Figure I8.22A). E. The mansupials, such as the American opossumand the kangaroo,give birth to embryonic young that completedevelopment in their mother'spouch. Most marsupialslive in Australia, on neighboring islands,and in Central and SouthAmerica (Figure 18.22E). NOTE: Both groups evolved in isolation from other mammalsin areasthat were part of the Pangaeansupercontinent, Gondwana (see Module 15.3). F. The eutherians (placentals),comprising nearly 95Voof all mammalianspecies, include dogs,cats, cows, rodents,bats, and whales.Their embryosare nurturedinside the mother by a placenh, m organ that includes both maternaland embryonicvascular tissue(Figure 18.22C). G. Humans are eutherianmammals belonging to the order Primates.The relationshipof humansto other primatesis discussedin Chapter19. H. Preview.'Unit V will covermany otherdetails of the lives of animals,emphasizing humans,mammals, and vertebrates,in that order.

IV. Phylogeny of the Animal Kingdorn Module 18.23 A phylogenetic tree gives animal diversity an evolutionaryperspective. A. Constructionof a phylogenetictree shouldbe basedon severallines of evidencesuch as molecular data, comparativeanatomy, embryonic development,cladistic analysis, and,of course,fossil records. B. Figure t8.23A convenientlyillusftates the major evolutionarysteps of the animal phyla in a phylogenetic tree using a traditional approach. 1. Like most phylogeneticfees, this one is hypotheticaland servesmainly to stimulate researchand focus discussion. 2. Colonial protists were the ancestorsof the animal kingdom. 3. Spongesmay be a separateline of evolution,or a very early sidelineage. 4. Two lines of tissue-levelanimals evolved: radially symmetricalanimals, represented by cnidarians, and bilaterally symmetricalanimals, the most primitive of which are representedby the flatworms. 5. Among the bilateralanimals, two lines evolved:those with pseudocoeloms,representedby the roundworms,and thosewith coeloms,the most primitive of which are representedby the mollusks. 6. Among the coelomateanimals, two lines evolved:those whose coelom developed from hollow outgrowthsof the digestivetube-the echinodermsand chordates (deuterostomes);and thosewhose coelom developed from solid massesof cells- the mollusks, annelids,and arthropods(protostomes). 186 Instructor's Guide to Textand Media

c. There is a debateregarding the developmentof segmentationand how often it evolved in animal evolution. l. Echinoderms,which are not segmented,and chordatesmay have diverged prior to segmentation;or segmentationmay havebeen lost in the echinodermsas they evolved radial symmetry. 2. Likewise, segmentationmay have evolvedin annelidsand arthropods after the mollusks diverged;or segmentationmay havebeen lost during the evolution of the mollusk lineage. 3. Segmentationitself may haveevolved independently in the deuterostome and pro- tostomelineages. 4. Molecular evidenceindicates that segmentationarose early in the history of animal evolution,possibly in the ancestorof all bilateralanimals. In this case segmentation, or its lack, may be due to genesthat affect early development. D. The molecular phylogenetictree is basedon nucleotide sequencedata from rRNA (Figure18.238). l. There are severalsimilarities betweenthe traditional tree and the molecular-based tree. For examplethe earliestbranching in both treesrecognizes those that have true tissuesand thosethat do not. Bilateral and radial symmetryis a branch point in both trees.Deuterostomes are recognizedin both trees. 2. One major differencein the molecular tree is that the protostomesare divided into two separatebranches: the Lophofrochozoahave similarities in molecular sequence data and are namedfor the feeding apparatusand larva. The other branch of the protostomes,the Ecdysozoa,share a commonfeature: They need to shed their exoskeleton. Review:Gene expression and embryonicdevelopment (Chapter 11).

Module lE.Z Connections:Humans threaten animal diversity by inroducing non-native species. A. The introduction of a nonnativespecies to a new environmentcan result in only two scenarios:The specieswill die, or it will surviveand havedevastating effects on the ecosysteminto whichit was introduced. B. Rabbits, foxes, and canetoads have been introducedto Australia and all three have endedin the secondscenario. c. Millions of dollars are now being spent eachyear by the Australian government in an effort to combatthe ecologicaldisaster caused by human intervention.

ClassActivities 1. Ask your studentsto considerthe advantages(and disadvantages)of cephalization.While this seems like a simplequestion, I find that studentsoften strugglewith developinga scientifically sound response. 2. To haveyour classgain a real appreciationof insect diversity,have your studentsstart an insect collection. -l

Chapter 18 The Evolution of Animal Diversity 187

TransparencyAcetates Figure18.18 Thelife cycleofa seastar Figure18.2 Colonialprotists Figure18.38 Radialsymmetry Figure 18.3C Structureand feeding of a simplesponge Figure18.38 A choanoflagellatecolony (about 0.02 mm high) Figure18.4D Cnidocyteaction Figure18.5 Bilateralsymmebry Figure18.6A A freeJivingflatworm, the planarian(most are about5-10 mm long) Figure18.68 A fluke (Schistosoma)and its life cycle(adults are about 1 cm long) Figure18.6C A tapeworm,a parasiticflatworm Figure18.74 No bodycavity (a flatworm) Figure 18.78 Pseudocoelom(a roundworm) Figure18.7C Truecoelom (an earthworm) Figure18.9A The generalbody plan of a mollusk Figure18.10A Segmentationin an earthworm Figure18.124 The structureofan arthropod,a lobster Figure18.134 Insectanatomy, as seenin a grasshopper Figure18.138 A damselfly Figure 18.13C A watersnider Figure18.13D A groundbeetle Figure18.138 A hawkmoth Figure18.13F A mosquito Figurel8.l3c A paperwasp Figure18.14A The watervascularsystem (canals and tube feet) ofa seastar Figurel8.l5A Tunicates Figure18.158 Lancelets(5-15 cm long) Figure 18.16 Characteristicsofvertebrates Figure18.17B The origin of vertebratejaws Figure18.18B Diagnosticfeatures of a bony fish Figure18.19D Skeletonof Acanthostega,aDevonian four-legged fish Figure18.20C A packof Deinonychzsattacking a largerdinosaur Figure18.214 Archaeopteryx,an extinctbird Figure18.23A A traditionalphylogenetic tree of the animalkingdom Figure18.238 A molecularphylogenetictree 188 Instractor's Guide to Textand Media

Media

See the beginning of this book for a complete description of all media available for instructors and stu- dents. Animations and videos are available in the Campbell Image Presentation Library. Media Activities and Thinking as a Scientist investigations are available on the student CD-ROM and web site. AnimationsandVideos FileName Hydra ReleasingSperm Video 18-04A-HydraSpermVideo-B. mov Hydra ReleasingSperm Video 18-04A-HydraSpermVideo- S.mov Jelly SwimmingVideo I 8-048-JellySwimmingVideo.B.mov Jelly SwimmingVideo I 8-048-JellySwimmingVideo-S.mov Thimble JelliesVideo I 8-048-ThimbleJellyVideo-B.mov Thimble JelliesVideo I 8-04B-ThimbleJellyVideo-S.mov Hydra Eating WaterFlea Time-LapseVideo I 8-MD-HydraEatingVideo-B.mov Hydra Eating WaterFlea Time-LapseVideo 18-04D-HydraEatingVideo-S.mov C. elegansCrawling Video 18 -08A-CelegansCrawlVideo-B.mov C. elegansCrawling Video I 8-08A-CelegansCrawlVideo-S.mov C. elegansEmbryo Development Time-Lapse Video I 8-08A-CelegansDevelVideo-S.mov SeaSlugs Video 18-09C- SeaSlugsVideo-B.mov SeaSlugs Video 18-09C-SeaSlugsVideo-S.mov LobsterMouth PartsClose Up Video I 8- I 2A-LobsterMouthVideo-B.mov LobsterMouth PartsClose Up Video I 8- I 2A-LobsterMouthVideo-S.mov Butterfly EmergingVideo I 8- I 3E-ButterflyVideo-B.mov Butterfly EmergingVideo I 8- 13E-Butterfl yVideo-S.mov EchinodermTirbe Feet Video I 8- 148-EchinTirbeFeetVideo-B.mov EchinodermTtrbe Feet Video I 8- 148-EchinTubeFeetVideo-S.mov Manta Ray Video 18- I 8A-MantaRayVideo-B.mov Manta Ray Video I 8- I 8A-MantaRayVideo-S.mov Bat Licking NectarVideo l8-22-BatLickingVideo-B.mov Bat Licking NectarVideo 78-22-BatlickingVideo-S.mov ActivitiesandThinking as a Scientist ModuleNumber Web/CDActivity l8A: Characteistics of Invenebrates 18.14 Web/CDActivity l8B: Characteristicsof Chordates t8.22 Web/CDActivity l8C: Aninal PhylogenertcTree 18.23 Web/CDThinking as a Scientist:How Are Insect Species Identified? 18.13 Web/CDThinking as a Scientist:How Does Bone Structure ShedLight on the Oigin of Birds? t8.2r Web/CDThinking as a Scientist:How Do Molecular Data Fit TraditionalPhylo genie s ? 18.23