Animal Development Chapter 44

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Part

XII

Animal Diversity

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874 Part XII Animal Diversity

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44

The NoncoelomateAnimals

Concept Outline

44.1 Animals are multicellular heterotrophs withoutcell walls.

Some General Features of Animals. Animals lack cellwalls and move more rapidly and in more complex waysthan other organisms. The animal kingdom is divided intoanimals without symmetry and tissues, and animals withsymmetry and tissues.

Five Key Transitions in Body Plan. Over the course ofanimal evolution, the animal body plan has undergonemany changes, five of them significant.

44.2 The simplest animals are not bilaterallysymmetrical.

Parazoa. Sponges are the most primitive animals, withouteither tissues or, for the most part, symmetry.

Eumetazoa: The Radiata. Cnidarians and ctenophoranshave distinct tissues and radial symmetry.

44.3 Acoelomates are solid worms that lack a bodycavity.

Eumetazoa: The Bilaterian Acoelomates. Flatwormsare the simplest bilaterally symmetrical animals; they lack abody cavity, but possess true organs.

44.4 Pseudocoelomates have a simple body cavity.

The Pseudocoelomates. Nematodes and rotifers possessa simple body cavity.

44.5 The coming revolution in animal taxonomy willlikely alter traditional phylogenies.

Reevaluating How the Animal Body Plan Evolved. Theuse of molecular data will most likely generate changes totraditional animal phylogenies.

We will now explore the great diversity of animals, theresult of a long evolutionary history. Animals, con-

stituting millions of species, are among the most abundantliving things. Found in every conceivable habitat, they be-wilder us with their diversity. We will start with the sim-plest members of the animal kingdomsponges, jellyfish,and simple worms. These animals lack a body cavity calleda coelom, and are thus called noncoelomates (figure 44.1).The major organization of the animal body first evolved inthese animals, a basic body plan upon which all the rest ofanimal evolution has depended. In chapters 45 through 48,we will consider the more complex animals. Despite theirgreat diversity, you will see that all animals have much in

common.

FIGURE 44.1A noncoelomate: a marine flatworm. Some of the earliestinvertebrates to evolve, marine flatworms possess internal organsbut lack a true cavity called a coelom.


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the zygote. Consequently, with a few exceptions, there isno counterpart among animals to the alternation of haploid(gametophyte) and diploid (sporophyte) generations char-

acteristic of plants (see chapter 32).

Embryonic Development. Most animals have a similarpattern of embryonic development. The zygote first under-goes a series of mitotic divisions, called cleavage, and be-comes a solid ball of cells, the morula, then a hollow ball ofcells, the blastula. In most animals, the blastula folds in-ward at one point to form a hollow sac with an opening atone end called the blastopore. An embryo at this stage is

called a gastrula. The subsequent growth and movementof the cells of the gastrula produce the digestive system,also called the gut or intestine. The details of embryonicdevelopment differ widely from one phylum of animals toanother and often provide important clues to the evolu-tionary relationships among them.

The Classification of Animals

Two subkingdoms are generally recognized within the

kingdom Animalia: Parazoaanimals that for the most partlack a definite symmetry and possess neither tissues nor or-gans, mostly comprised of the sponges, phylum Porifera;and Eumetazoaanimals that have a definite shape andsymmetry and, in most cases, tissues organized into organsand organ systems. Although very different in structure,both types evolved from a common ancestral form (figure44.2) and possess the most fundamental animal traits.

All eumetazoans form distinct embryonic layers during

development that differentiate into the tissues of the adultanimal. Eumetazoans of the subgroup Radiata (having ra-dial symmetry) have two layers, an outer ectodermand aninner endoderm, and thus are called diploblastic. Allother eumetazoans, the Bilateria (having bilateral symme-try), are triploblastic and produce a third layer, the meso-derm, between the ectoderm and endoderm. No such lay-ers are present in sponges.

The major phyla of animals are listed in table 44.1. Thesimplest invertebrates make up about 14 phyla. In this

chapter, we will discuss 8 of these 14 phyla and focus in de-tail on 4 major phyla: phylum Porifera (sponges), whichlacks any tissue organization; phylum Cnidaria (radiallysymmetrical jellyfish, hydroids, sea anemones, and corals);phylum Platyhelminthes (bilaterally symmetrical flat-worms); and phylum Nematoda (nematodes), a phylum thatincludes both free-living and parasitic roundworms.

Animals are complex multicellular organisms typicallycharacterized by high mobility and heterotrophy. Mostanimals also possess internal tissues and organs andreproduce sexually.


Some General Features of Animals

Animals are the eaters or consumers of the earth. They are

heterotrophs and depend directly or indirectly on plants,photosynthetic protists (algae), or autotrophic bacteria fornourishment. Animals are able to move from place to placein search of food. In most, ingestion of food is followed bydigestion in an internal cavity.

Multicellular Heterotrophs. All animals are multicellu-lar heterotrophs. The unicellular heterotrophic organismscalled Protozoa, which were at one time regarded as simple

animals, are now considered to be members of the kingdomProtista, the large and diverse group we discussed in chap-ter 35.

Diverse in Form. Almost all animals (99%) are inverte-brates, lacking a backbone. Of the estimated 10 million liv-ing animal species, only 42,500 have a backbone and are re-ferred to asvertebrates.Animals are very diverse in form,ranging in size from ones too small to see with the nakedeye to enormous whales and giant squids. The animal king-

dom includes about 35 phyla, most of which occur in thesea. Far fewer phyla occur in fresh water and fewer stilloccur on land. Members of three phyla, Arthropoda (spi-ders and insects), Mollusca (snails), and Chordata (verte-brates), dominate animal life on land.

No Cell Walls. Animal cells are distinct among multicel-lular organisms because they lack rigid cell walls and areusually quite flexible. The cells of all animals but sponges

are organized into structural and functional units called tis-sues, collections of cells that have joined together and arespecialized to perform a specific function; muscles andnerves are tissues types, for example.

Active Movement. The ability of animals to move morerapidly and in more complex ways than members of otherkingdoms is perhaps their most striking characteristic andone that is directly related to the flexibility of their cellsand the evolution of nerve and muscle tissues. A remark-

able form of movement unique to animals is flying, an abil-ity that is well developed among both insects and verte-brates. Among vertebrates, birds, bats, and pterosaurs(now-extinct flying reptiles) were or are all strong fliers.The only terrestrial vertebrate group never to have had fly-ing representatives is amphibians.

Sexual Reproduction. Most animals reproduce sexually.Animal eggs, which are nonmotile, are much larger than

the small, usually flagellated sperm. In animals, cellsformed in meiosis function directly as gametes. The hap-loid cells do not divide by mitosis first, as they do in plantsand fungi, but rather fuse directly with each other to form

44.1 Animals are multicellular heterotrophs without cell walls.


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Chapter 44 The Noncoelomate Animals 877

?

Radiata(CnidariaandCteno

phora)

Porifera(sp

onges)

Platyhelminthes(flatw

orms)

Nematoda(roundw

orms)

Rotifera(rotifers)

Mollusca

Annelida

Arthropoda

Lophoph

orates

Echinodermata

Ch

ordata

Ancestral protist

Multicellularity

Tissues

Bilateral symmetry

Body cavity

Coelom

Deuterostome development,Endoskeleton

Notochord,Segmentation,Jointedappendages

Jointed appendages,Exoskeleton

Segmentation

Protostome development

Pseudocoel

No body cavity

Radial symmetry

No true tissues

Parazoa

Radiata

Acoelomates Pseudocoelomates

Eumetazoa

Bilateria

Protostomes

Segmented

Coelomates

Deuterostomes

Segmented

1

1

2

2

3

3

4

4

5

5

FIGURE 44.2A possible phylogeny of the major groups of the kingdom Animalia.Transitions in the animal body plan are identified along thebranches; the five key advances are the evolution of tissues, bilateral symmetry, a body cavity, protostome and deuterostome development,and segmentation.


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Table 44.1 The Major Animal Phyla

ApproximateNumber of

Phylum Typical Examples Key Characteristics Named Species

Arthropoda(arthropods)

Mollusca

(mollusks)

Chordata(chordates)

Platyhelminthes(flatworms)

Nematoda(roundworms)

Annelida(segmentedworms)

Beetles, otherinsects, crabs,spiders

Snails, oysters,

octopuses,nudibranchs

Mammals, fish,reptiles, birds,

amphibians

Planaria,tapeworms,liver flukes

Ascaris, pinworms,hookworms,Filaria

Earthworms,polychaetes,beach tubeworms,leeches

Most successful of all animal phyla;chitinous exoskeleton covering segmentedbodies with paired, jointed appendages;many insect groups have wings

Soft-bodied coelomates whose bodies are

divided into three parts: head-foot, visceralmass, and mantle; many have shells; almostall possess a unique rasping tongue, called aradula; 35,000 species are terrestrial

Segmented coelomates with a notochord;possess a dorsal nerve cord, pharyngeal

slits, and a tail at some stage of life; invertebrates, the notochord is replacedduring development by the spinal column;20,000 species are terrestrial

Solid, unsegmented, bilaterally symmetricalworms; no body cavity; digestive cavity, ifpresent, has only one opening

Pseudocoelomate, unsegmented, bilaterallysymmetrical worms; tubular digestive tractpassing from mouth to anus; tiny; withoutcilia; live in great numbers in soil andaquatic sediments; some are important

animal parasites

Coelomate, serially segmented, bilaterallysymmetrical worms; complete digestivetract; most have bristles called setae oneach segment that anchor them duringcrawling

1,000,000

110,000

42,500

20,000

12,000+

12,000


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Table 44.1 The Major Animal Phyla (continued)



Cnidaria(cnidarians)

Echinodermata(echinoderms)

Porifera(sponges)

Bryozoa(moss animals)

Rotifera(wheel animals)

Jellyfish, hydra,corals, seaanemones

Sea stars, seaurchins, sand

dollars, seacucumbers

Barrel sponges,boring sponges,basket sponges,vase sponges

Bowerbankia,Plumatella, seamats, sea moss

Rotifers

Soft, gelatinous, radially symmetricalbodies whose digestive cavity has a singleopening; possess tentacles armed withstinging cells called cnidocytes that shootsharp harpoons called nematocysts; almostentirely marine

Deuterostomes with radially symmetricaladult bodies; endoskeleton of calcium

plates; five-part body plan and uniquewater vascular system with tube feet; ableto regenerate lost body parts; marine

Asymmetrical bodies without distincttissues or organs; saclike body consists oftwo layers breached by many pores;internal cavity lined with food-filteringcells called choanocytes; most marine (150species live in fresh water)

Microscopic, aquatic deuterostomes thatform branching colonies, possess circularor U-shaped row of ciliated tentacles forfeeding called a lophophore that usuallyprotrudes through pores in a hardexoskeleton; also called Ectoproctabecause the anus or proct is external to thelophophore; marine or freshwater

Small, aquatic pseudocoelomates with acrown of cilia around the mouthresembling a wheel; almost all live infresh water

10,000

6,000

5,150

4,000

2,000


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Table 44.1 The Major Animal Phyla (continued)



Five phyla ofminor worms

Brachiopoda(lamp shells)

Velvet worms, acornworms, arrow worms,giant tube worms

Comb jellies, seawalnuts

Phoronis

Nanaloricusmysticus

Chaetognatha (arrow worms): coelomatedeuterostomes; bilaterally symmetrical;large eyes (some) and powerful jaws

Hemichordata (acorn worms): marineworms with dorsal andventral nerve cords

Onychophora (velvet worms):protostomes with a chitinous exoskeleton;evolutionary relics

Pogonophora (tube worms): sessile deep-

sea worms with long tentacles; live withinchitinous tubes attached to the ocean floor

Nemertea (ribbon worms): acoelomate,bilaterally symmetrical marine worms withlong extendable proboscis

Like bryozoans, possess a lophophore, butwithin two clamlike shells; more than30,000 species known as fossils

Lingula

Ctenophora(sea walnuts)

Phoronida(phoronids)

Loricifera(loriciferans)

Gelatinous, almost transparent, oftenbioluminescent marine animals; eightbands of cilia; largest animals that usecilia for locomotion; complete digestive

tract with anal pore

Lophophorate tube worms; often live indense populations; unique U-shaped gut,instead of the straight digestive tube ofother tube worms

Tiny, bilaterally symmetrical, marinepseudocoelomates that live in spacesbetween grains of sand; mouthparts includea unique flexible tube; a recently discoveredanimal phylum (1983)

980

250

100

12

6


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Five Key Transitions in Body Plan

1. Evolution of Tissues

The simplest animals, the Parazoa, lack both defined tis-sues and organs. Characterized by the sponges, these ani-

mals exist as aggregates of cells with minimal intercellularcoordination. All other animals, the Eumetazoa, have dis-tinct tissues with highly specialized cells. The evolution oftissues is the first key transition in the animal body plan.

2. Evolution of Bilateral Symmetry

Sponges also lack any definite symmetry, growing asym-metrically as irregular masses. Virtually all other animalshave a definite shape and symmetry that can be definedalong an imaginary axis drawn through the animals body.Animals with symmetry belong to either the Radiata, ani-mals with radial symmetry, or the Bilateria, animals withbilateral symmetry.

Radial Symmetry. Symmetrical bodies first evolved inmarine animals belonging to two phyla: Cnidaria (jellyfish,sea anemones, and corals) and Ctenophora (comb jellies).The bodies of members of these two phyla, the Radiata, ex-

hibitradial symmetry, a body design in which the parts ofthe body are arranged around a central axis in such a waythat any plane passing through the central axis divides theorganism into halves that are approximate mirror images(figure 44.3a).

Bilateral Symmetry. The bodies of all other animals, theBilateria, are marked by a fundamental bilateral symme-try, a body design in which the body has a right and a lefthalf that are mirror images of each other (figure 44.3

b). A

bilaterally symmetrical body plan has a top and a bottom,better known respectively as the dorsaland ventralportionsof the body. It also has a front, or anteriorend, and a back,or posteriorend. In some higher animals like echinoderms(starfish), the adults are radially symmetrical, but even inthem the larvae are bilaterally symmetrical.

Bilateral symmetry constitutes the second major evolu-tionary advance in the animal body plan. This unique formof organization allows parts of the body to evolve in differ-

ent ways, permitting different organs to be located in dif-ferent parts of the body. Also, bilaterally symmetrical ani-mals move from place to place more efficiently thanradially symmetrical ones, which, in general, lead a sessileor passively floating existence. Due to their increased mo-bility, bilaterally symmetrical animals are efficient in seek-ing food, locating mates, and avoiding predators.

During the early evolution of bilaterally symmetrical an-imals, structures that were important to the organism in

monitoring its environment, and thereby capturing prey oravoiding enemies, came to be grouped at the anterior end.Other functions tended to be located farther back in thebody. The number and complexity of sense organs are

much greater in bilaterally symmetrical animals than theyare in radially symmetrical ones.

Much of the nervous system in bilaterally symmetricalanimals is in the form of major longitudinal nerve cords. Ina very early evolutionary advance, nerve cells becamegrouped around the anterior end of the body. These nervecells probably first functioned mainly to transmit impulsesfrom the anterior sense organs to the rest of the nervoussystem. This trend ultimately led to the evolution of a defi-nite head and brain area, a process called cephalization, aswell as to the increasing dominance and specialization ofthese organs in the more advanced animal phyla.


(a)

(b)

Ventral

Dorsal

Anterior

Posterior

Frontalplane

Sagittal

plane

Transverseplane

FIGURE 44.3A comparison of radial and bilateral symmetry. (a) Radiallysymmetrical animals, such as this sea anemone, can be bisectedinto equal halves in any two-dimensional plane. (b) Bilaterallysymmetrical animals, such as this squirrel, can only be bisectedinto equal halves in one plane (the sagittal plane).


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3. Evolution of a Body Cavity

A third key transition in the evolutionof the animal body plan was the evolu-tion of the body cavity. The evolutionof efficient organ systems within the

animal body was not possible until abody cavity evolved for supporting or-gans, distributing materials, and foster-ing complex developmental interac-tions.

The presence of a body cavity allowsthe digestive tract to be larger andlonger. This longer passage allows forstorage of undigested food, longer ex-

posure to enzymes for more completedigestion, and even storage and finalprocessing of food remnants. Such anarrangement allows an animal to eat agreat deal when it is safe to do so andthen to hide during the digestive process, thus limiting theanimals exposure to predators. The tube within the bodycavity architecture is also more flexible, thus allowing theanimal greater freedom to move.

An internal body cavity also provides space within which

the gonads (ovaries and testes) can expand, allowing the ac-cumulation of large numbers of eggs and sperm. Such stor-age capacity allows the diverse modifications of breedingstrategy that characterize the more advanced phyla of ani-mals. Furthermore, large numbers of gametes can be storedand released when the conditions are as favorable as possi-ble for the survival of the young animals.

Kinds of Body Cavities. Three basic kinds of body plans

evolved in the Bilateria.Acoelomates have no body cavity.Pseudocoelomates have a body cavity called the pseudo-coel located between the mesoderm and endoderm. A thirdway of organizing the body is one in which the fluid-filledbody cavity develops not between endoderm and meso-derm, but rather entirely within the mesoderm. Such abody cavity is called a coelom, and animals that possesssuch a cavity are called coelomates. In coelomates, the gutis suspended, along with other organ systems of the animal,within the coelom; the coelom, in turn, is surrounded by a

layer of epithelial cells entirely derived from the mesoderm.The portion of the epithelium that lines the outer wall ofthe coelom is called the parietal peritoneum, and the por-tion that covers the internal organs suspended within thecavity is called thevisceral peritoneum(figure 44.4).

The development of the coelom poses a problemcir-culationsolved in pseudocoelomates by churning thefluid within the body cavity. In coelomates, the gut isagain surrounded by tissue that presents a barrier to dif-

fusion, just as it was in solid worms. This problem issolved among coelomates by the development of a circu-latory system, a network of vessels that carries fluids toparts of the body. The circulating fluid, or blood, carries

nutrients and oxygen to the tissues and removes wastesand carbon dioxide. Blood is usually pushed through thecirculatory system by contraction of one or more muscu-lar hearts. In an open circulatory system, the bloodpasses from vessels into sinuses, mixes with body fluid,

and then reenters the vessels later in another location. Ina closed circulatory system, the blood is physically sep-arated from other body fluids and can be separately con-trolled. Also, blood moves through a closed circulatorysystem faster and more efficiently than it does through anopen system.

The evolut ionary re la tionship among coelomates ,pseudocoelomates, and acoelomates is not clear. Acoelo-mates, for example, could have given rise to coelomates,but scientists also cannot rule out the possibility thatacoelomates were derived from coelomates. The differentphyla of pseudocoelomates form two groups that do notappear to be closely related.

Advantages of a Coelom. What is the functional dif-ference between a pseudocoel and a coelom? The answerhas to do with the nature of animal embryonic develop-ment. In animals, development of specialized tissues in-volves a process called primary induction in which one

of the three primary tissues (endoderm, mesoderm, andectoderm) interacts with another. The interaction re-quires physical contact. A major advantage of the coelo-mate body plan is that it allows contact between meso-derm and endoderm, so that primary induction can occurduring development. For example, contact betweenmesoderm and endoderm permits localized portions ofthe digestive tract to develop into complex, highly spe-cialized regions like the stomach. In pseudocoelomates,

mesoderm and endoderm are separated by the body cav-ity, limiting developmental interactions between thesetissues that ultimately limits tissue specialization anddevelopment.


Ectoderm

Ectoderm

Mesoderm

Endoderm

Pseudocoel

Ectoderm

Mesoderm

Visceralperitoneum

Coelomiccavity

Endoderm

Parietalperitoneum

Mesoderm

Endoderm

Acoelomate Pseudocoelomate Coelomate

FIGURE 44.4Three body plans for bilaterally symmetrical animals.


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4. The Evolution of Protostome andDeuterostome Development

Two outwardly dis similar large phyla, Echinodermata(starfish) and Chordata (vertebrates), together with twosmaller phyla, have a series of key embryological features

different from those shared by the other animal phyla. Be-cause it is extremely unlikely that these features evolvedmore than once, it is believed that these four phyla share acommon ancestry. They are the members of a group calledthe deuterostomes.Members of the other coelomate ani-mal phyla are called protostomes. Deuterostomes evolvedfrom protostomes more than 630 million years ago.

Deuterostomes, like protostomes, are coelomates. Theydiffer fundamentally from protostomes, however, in the

way in which the embryo grows. Early in embryonicgrowth, when the embryo is a hollow ball of cells, a por-tion invaginates inward to form an opening called theblastopore. The blastopore of a protostome becomes theanimals mouth, and the anus develops at the other end. Ina deuterostome, by contrast, the blastopore becomes theanimals anus, and the mouth develops at the other end(figure 44.5).

Deuterostomes differ in many other aspects of embryogrowth, including the plane in which the cells divide. Per-

haps most importantly, the cells that make up an embry-onic protostome each contain a different portion of theregulatory signals present in the egg, so no one cell of theembryo (or adult) can develop into a complete organism. Inmarked contrast, any of the cells of a deuterostome can de-velop into a complete organism.

5. The Evolution of Segmentation

The fifth key transition in the animal body plan involvedthe subdivision of the body into segments.Just as it is effi-cient for workers to construct a tunnel from a series of

identical prefabricated parts, so segmented animals are as-sembled from a succession of identical segments. Duringthe animals early development, these segments becomemost obvious in the mesoderm but later are reflected in theectoderm and endoderm as well. Two advantages resultfrom early embryonic segmentation:

1. In annelids and other highly segmented animals, eachsegment may go on to develop a more or less com-plete set of adult organ systems. Damage to any onesegment need not be fatal to the individual becausethe other segments duplicate that segments func-tions.

2. Locomotion is far more effective when individualsegments can move independently because the animalas a whole has more flexibility of movement. Becausethe separations isolate each segment into an individ-ual skeletal unit, each is able to contract or expandautonomously in response to changes in hydrostaticpressure. Therefore, a long body can move in waysthat are often quite complex.

Segmentation, also referred to as metamerism, underliesthe organization of all advanced animal body plans. Insome adult arthropods, the segments are fused, but seg-mentation is usually apparent in their embryological devel-opment. In vertebrates, the backbone and muscular areasare segmented, although segmentation is often disguised inthe adult form. True segmentation is found in only threephyla: the annelids, the arthropods, and the chordates, al-though this trend is evident in many phyla.

Five key transitions in body design are responsible formost of the differences we see among the major animal

phyla: the evolution of (1) tissues, (2) bilateralsymmetry, (3) a body cavity, (4) protostome anddeuterostome development, and (5) segmentation.

Invagination inearly embryo

Invagination inearly embryo

Blastopore

Anus

AnusMouth

Mouth

(a) Protostomes (b) Deuterostomes

Blastopore

Ectoderm

Mesodermcells

Endoderm

Ectoderm

Endoderm

Mesoderm

FIGURE 44.5The fate of the blastopore. (a) In protostomes, the blastopore becomes the animals mouth. (b) In deuterostomes, the blastoporebecomes the animals anus.


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Parazoa

The sponges are Parazoans, animals

that lack tissues and organs and a defi-nite symmetry. However, sponges,like all animals, have true, complexmulticellularity, unlike their protistanancestors. The body of a sponge con-tains several distinctly different typesof cells whose activities are loosely co-ordinated with one another. As wewill see, the coordination between celltypes in the eumetazoans increasesand becomes quite complex.

The Sponges

There are perhaps 5000 species ofmarine sponges, phylum Porifera,and about 150 species that live infresh water. In the sea, sponges areabundant at all depths. Although

some sponges are tiny, no more thana few millimeters across, some, likethe loggerhead sponges, may reach 2meters or more in diameter. A fewsmall ones are radially symmetrical,but most members of this phylumcompletely lack symmetry. Manysponges are colonial. Some have alow and encrusting form, while oth-ers may be erect and lobed, some-times in complex patterns. Althoughlarval sponges are free-swimming,adults are sessile, or anchored inplace to submerged objects.

Sponges, like all animals, arecomposed of multiple cell types (seefigure 44.7), but there is relativelylittle coordination among spongecells. A sponge seems to be little

more than a mass of cells embeddedin a gelatinous matrix, but thesecells recognize one another with ahigh degree of fidelity and are specialized for differentfunctions of the body.

The basic structure of a sponge can best be understoodby examining the form of a young individual. A small,anatomically simple sponge first attaches to a substrateand then grows into a vaselike shape. The walls of the

vase have three functional layers. First, facing into theinternal cavity are specialized flagellated cells calledchoanocytes, or collar cells. These cells line either theentire body cavity or, in many large and more complex

sponges, specialized chambers. Sec-ond, the bodies of sponges arebounded by an outer epithelial layer

consisting of flattened cells some-what like those that make up the ep-ithelia, or outer layers, of other ani-mal phyla. Some portions of thislayer contract when touched or ex-posed to appropriate chemical stim-uli, and this contraction may causesome of the pores to close. Third,between these two layers, spongesconsist mainly of a gelatinous, pro-tein-rich matrix called the mesohyl,within which various types of amoe-boid cells occur. In addition, manykinds of sponges have minute nee-dles of calcium carbonate or silicaknown as spicules, or fibers of atough protein called spongin, orboth, within this matrix. Spiculesand spongin strengthen the bodies of

the sponges in which they occur. Aspongin skeleton is the model for thebathtub sponge, once the skeleton ofa real animal, but now largely knownfrom its cellulose and plastic mimics.

Sponges feed in a unique way.The beating of flagella that line theinside of the sponge draws water inthrough numerous small pores; the

name of the phylum, Porifera, refersto this system of pores. Plankton andother small organisms are filteredfrom the water, which flows throughpassageways and eventually is forcedout through an osculum, a special-ized, larger pore (figure 44.6).

Choanocytes. Each choanocyteclosely resembles a protist with a sin-

gle flagellum (figure 44.7), a similaritythat reflects its evolutionary deriva-tion. The beating of the flagella of the

many choanocytes that line the body cavity draws water inthrough the pores and through the sponge, thus bringingin food and oxygen and expelling wastes. Each choanocyteflagellum beats independently, and the pressure they createcollectively in the cavity forces water out of the osculum. Insome sponges, the inner wall of the body cavity is highly

convoluted, increasing the surface area and, therefore, thenumber of flagella that can drive the water. In such asponge, 1 cubic centimeter of sponge can propel more than20 liters of water a day.


44.2 The simplest animals are not bilaterally symmetrical.

Sponges

Cnidarians

Flatworms

Nematodes

Mollusks

Annelids

Arthropods

Echinoderms

Chordates

FIGURE 44.6Aplysina longissima.This beautiful, brightblue and yellow elongated sponge is foundon deep regions of coral reefs. The osculaare ringed with yellow.


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Reproduction in Sponges. Some sponges will re-formthemselves once they have passed through a silk mesh.Thus, as you might suspect, sponges frequently repro-duce by simply breaking into fragments. If a spongebreaks up, the resulting fragments usually are able to re-constitute whole new individuals. Sexual reproduction isalso exhibited by sponges, with some mature individuals

producing eggs and sperm. Larval sponges may undergotheir initial stages of development within the parent.They have numerous external, flagellated cells and arefree-swimming. After a short planktonic stage, they settle

down on a suitable substrate, where they begin theirtransformation into adults.

Sponges probably represent the most primitive animals,possessing multicellularity but neither tissue-leveldevelopment nor body symmetry. Their cellularorganization hints at the evolutionary ties between the

unicellular protists and the multicellular animals.Sponges are unique in the animal kingdom inpossessing choanocytes, special flagellated cells whosebeating drives water through the body cavity.


The body of a spongeis lined with cells calledchoanocytes and isperforated by many tinypores through whichwater enters.

Sponges are multicellular,containing many different cell

types, such as amoebocytesand choanocytes.

The beating flagella of the many choanocytesdraw water in through the pores, through thesponge, and eventually out through theosculum.

When a choanocyte beats its flagellum,water is drawn down through openings inits collar, where food particles becometrapped. The particles are then devouredby endocytosis.

Each choanocyte is exactlylike a type of unicellularprotist called achoanoflagellate. Itseems certain that theseprotists are the ancestors

of the sponges, andprobably of all animals.

Between the outer wall and the bodycavity of the sponge body are amoeboid

cells called amoebocytes that secrete hardmineral needles called spicules and toughprotein fibers called spongin. These structuresstrengthen and protect the sponge.

Osculum

Pore

Water

Pore

Spicule

Spongin

Choanocyte

Choanocyte

Collar

Flagellum

Nucleus

Amoebocyte

Epithelialwall

PHYLUM PORIFERA: Multicellularity

FIGURE 44.7The body of a sponge is multicellular.The first evolutionary advance seen in animals is complex multicellularity, in which individualsare composed of many highly specialized kinds of cells.


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Eumetazoa: TheRadiata

The subkingdom Eumetazoa containsanimals that evolved the first key

transition in the animal body plan:distincttissues.Two distinct cell layersform in the embryos of these animals:an outer ectoderm and an inner endo-derm. These embryonic tissues giverise to the basic body plan, differenti-ating into the many tissues of theadult body. Typically, the outer cov-ering of the body (called the epider-mis) and the nervous system develop

from the ectoderm, and the layer ofdigestive tissue (called the gastroder-mis) develops from the endoderm. Alayer of gelatinous material, called themesoglea, lies between the epidermisand gastrodermis and contains themuscles in most eumetazoans.

Eumetazoans also evolved truebody symmetry and are divided into

two major groups. The Radiata in-cludes two phyla of radially symmet-rical organisms, Cnidaria (pro-nounced ni-DAH-ree-ah), thecnidarianshydroids, jellyfish, seaanemones, and coralsandCtenophora (pronounced tea-NO-fo-rah), the comb jellies, or ctenophores.All other eumetazoans are in the Bila-teria and exhibit a fundamental bilat-

eral symmetry.

The Cnidarians

Cnidarians are nearly all marine, al-though a few live in fresh water.These fascinating and simply con-structed animals are basically gelati-nous in composition. They differ

markedly from the sponges in organi-zation; their bodies are made up ofdistinct tissues, although they have not evolved true organs.These animals are carnivores. For the most part, they do notactively move from place to place, but rather capture theirprey (which includes fishes, crustaceans, and many otherkinds of animals) with the tentacles that ring their mouths.

Cnidarians may have two basic body forms, polyps andmedusae (figure 44.8). Polyps are cylindrical and are usu-ally found attached to a firm substrate. They may be soli-

tary or colonial. In a polyp, the mouth faces away from thesubstrate on which the animal is growing, and, therefore,often faces upward. Many polyps build up a chitinous or

calcareous (made up of calcium car-bonate) external or internal skeleton,or both. Only a few polyps are free-floating. In contrast, most medusaeare free-floating and are often um-brella-shaped. Their mouths usuallypoint downward, and the tentacleshang down around them. Medusae,particularly those of the classScyphozoa, are commonly known asjel lyfish because their mesoglea isthick and jellylike.

Many cnidarians occur only aspolyps, while others exist only asmedusae; still others alternate be-

tween these two phases during theirlife cycles. Both phases consist ofdiploid individuals. Polyps may re-produce asexually by budding; if theydo, they may produce either newpolyps or medusae. Medusae repro-

duce sexually. In most cnidarians, fer-tilized eggs give rise to free-swim-ming, multicellular, ciliated larvaeknown as planulae. Planulae are com-mon in the plankton at times and maybe dispersed widely in the currents.

A major evolutionary innovation incnidarians, compared with sponges, isthe internal extracellular digestion offood (figure 44.9). Digestion takesplace within a gut cavity, rather thanonly within individual cells. Digestiveenzymes are released from cells lining

the walls of the cavity and partiallybreak down food. Cells lining the gutsubsequently engulf food fragmentsby phagocytosis.

The extracellular fragmentationthat precedes phagocytosis and intra-cellular digestion allows cnidarians todigest animals larger than individualcells, an important improvement

over the strictly intracellular diges-tion of sponges.Nets of nerve cells coordinate contraction of cnidarian

muscles, apparently with little central control. Cnidarianshave no blood vessels, respiratory system, or excretoryorgans.

On their tentacles and sometimes on their body surface,cnidarians bear specialized cells called cnidocytes.The nameof the phylum Cnidaria refers to these cells, which are highlydistinctive and occur in no other group of organisms. Withineach cnidocyte is a nematocyst, a small but powerful har-poon. Each nematocyst features a coiled, threadlike tube.Lining the inner wall of the tube is a series of barbed spines.


Sponges

Cnida

rians

Flatw

orms

Nema

todes

Mollusks

Annelids

Arthropods

Echinod

erms

Chor

dates

Gastrovascularcavity

Gastrovascular cavity

Mouth

Epidermis

Mesoglea

Gastrodermis

Tentacles

Polyp

Medusa

FIGURE 44.8Two body forms of cnidarians, themedusa and the polyp.These two phasesalternate in the life cycles of manycnidarians, but a numberincluding thecorals and sea anemones, for exampleexist only as polyps. Both forms have twofundamental layers of cells, separated by ajellylike layer called the mesoglea.


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Cnidarians use the threadlike tube to spear their prey andthen draw the harpooned prey back with the tentacle contain-ing the cnidocyte. Nematocysts may also serve a defensivepurpose. To propel the harpoon, the cnidocyte uses waterpressure. Before firing, the cnidocyte builds up a very high in-ternal osmotic pressure. This is done by using active transportto build a high concentration of ions inside, while keeping itswall impermeable to water. Within the undischarged nemato-cyst, osmotic pressure reaches about 140 atmospheres.

When a flagellum-like trigger on the cnidocyte is stim-ulated to discharge, its walls become permeable to water,which rushes inside and violently pushes out the barbedfilament. Nematocyst discharge is one of the fastest cellu-lar processes in nature. The nematocyst is pushed out-ward so explosively that the barb can penetrate even thehard shell of a crab. A toxic protein often produced astinging sensation, causing some cnidarians to be calledstinging nettles.


PHYLUM CNIDARIA: Tissues and radial symmetry

Hydra

Cross-section

Stinging cell (cnidocyte)with nematocyst Trigger

Discharged

nematocyst

Undischargednematocyst

Filament

Tentacles

Hydra and other jellyfish are radiallysymmetrical, with parts arranged around a

central axis like petals of a daisy.

The cells of cnidarians are organized into tissues.

A major innovation of hydra and jellyfish is extracellulardigestion of foodthat is, digestion within a gut cavity.

Tentacles and body have stingingcells (cnidocytes) that contain smallbut very powerful harpoons callednematocysts.

The harpoon is propelled by osmoticpressure and is one of the fastest andmost powerful processes in nature.

Hydra use nematocysts tospear prey and then draw thewounded prey back to the hydra.

The barb explodes out of thestinging cell at a high velocity andcan even penetrate the hard shellof a crustacean.

Hydra and jellyfish arecarnivores that capture theirprey with tentacles that ringtheir mouth.

Mouth

Sensory cell

Gastrodermis

Mesoglea

Epidermis

Cnidocyte

FIGURE 44.9Eumetazoans all have tissues and symmetry.The cells of a cnidarian like thisHydra are organized into specialized tissues. The interior

gut cavity is specialized for extracellular digestionthat is, digestion within a gut cavity rather than within individual cells. Cnidarians arealso radially symmetrical.


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Classes of Cnidarians

There are four classes of cnidarians: Hydrozoa (hydroids),Scyphozoa (jellyfish), Cubozoa (box jellyfish), and Antho-zoa (anemones and corals).

Class Hydrozoa: The Hydroids. Most of the approxi-mately 2700 species of hydroids (class Hydrozoa) have bothpolyp and medusa stages in their life cycle (figure 44.10).Most of these animals are marine and colonial , such asObelia and the already mentioned, very unusual Por-tuguese man-of-war. Some of the marine hydroids are bio-luminescent.

A well-known hydroid is the abundant freshwater genusHydra,which is exceptional in that it has no medusa stageand exists as a solitary polyp. Each polyp sits on a basal

disk, which it can use to glide around, aided by mucous se-cretions. It can also move by somersaulting, bending overand attaching itself to the substrate by its tentacles, andthen looping over to a new location. If the polyp detachesitself from the substrate, it can float to the surface.

Class Scyphozoa: The Jellyfish. The approximately 200species of jellyfish (class Scyphozoa) are transparent ortranslucent marine organisms, some of a striking orange,blue, or pink color (figure 44.11). These animals spendmost of their time floating near the surface of the sea. In allof them, the medusa stage is dominantmuch larger andmore complex than the polyps. The medusae are bell-

shaped, with hanging tentacles around their margins. Thepolyp stage is small, inconspicuous, and simple in structure.

The outer layer, or epithelium, of a jellyfish contains anumber of specialized epitheliomuscular cells, each ofwhich can contract individually. Together, the cells form amuscular ring around the margin of the bell that pulsesrhythmically and propels the animal through the water. Jel-lyfish have separate male and female individuals. After fer-tilization, planulae form, which then attach and developinto polyps. The polyps can reproduce asexually as well asbudding off medusae. In some jellyfish that live in the openocean, the polyp stage is suppressed, and planulae developdirectly into medusae.


Feedingpolyp

Medusa bud

Reproductivpolyp

e

OvaryMedusae

Eggs Sperm

Sexualreproduction

Zygote

Testis

Blastula

Free-swimmingplanula larva

Settles down tostart new colony

Young colony andasexual budding

Maturecolony

FIGURE 44.10The life cycle ofObelia, a marinecolonial hydroid. Polyps reproduce byasexually budding, forming colonies. Theymay also give rise to medusae, whichreproduce sexually via gametes. Thesegametes fuse, producing zygotes thatdevelop into planulae, which, in turn, settle

down to produce polyps.

FIGURE 44.11Class Scyphozoa.Jellyfish,Aurelia aurita.


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Class Cubozoa: The Box Jellyfish.Until recently the cubozoa were con-sidered an order of Scyphozoa. Astheir name implies, they are box-shaped medusa (the polyp stage is in-conspicuous and in many cases not

known). Most are only a few cm inheight, although some are 25 cm tall.A te ntac le or group of tentac le s isfound at each corner of the box (figure44.12). Box jellies are strong swim-mers and voracious predators of fish.Stings of some species can be fatal tohumans.

Class Anthozoa: The Sea Anemones

and Corals. By far the largest class ofcnidarians is Anthozoa, the floweranimals (from the Greek anthos,meaning flower). The approxi-mately 6200 species of this group aresolitary or colonial marine animals.They inc lude stonelike corals, sof t-bodied sea anemones, and othergroups known by such fanciful names

as sea pens, sea pansies, sea fans, andsea whips (figure 44.13). All of thesenames reflect a plantlike body toppedby a tuft or crown of hollow tentacles.Like other cnidarians, anthozoans usethese tentacles in feeding. Nearly allmembers of this class that live in shal-low waters harbor symbiotic algae,which supplement the nutr it ion oftheir hosts through photosynthesis.Fertilized eggs of anthozoans usuallydevelop into planulae that settle anddevelop into polyps; no medusae areformed.

Sea anemones are a large group ofsoft-bodied anthozoans that live incoastal waters all over the world andare especially abundant in the tropics.When touched, most sea anemones re-

tract their tentacles into their bodiesand fold up. Sea anemones are highlymuscular and relatively complex organ-isms, with greatly divided internal cavi-ties. These animals range from a fewmillimeters to about 10 centimeters indiameter and are perhaps twice thathigh.

Corals are another major group ofanthozoans. Many of them secretetough outer skeletons, or exoskele-tons, of calcium carbonate and arethus stony in texture. Others, includ-

ing the gorgonians, or soft corals, donot secrete exoskeletons. Some of thehard corals help form coral reefs,which are shal low-water li mestoneridges that occur in warm seas. Al-though the waters where coral reefs

develop are often nutrient-poor, thecoral animals are able to grow activelybecause of the abundant algae foundwithin them.

The Ctenophorans (CombJellies)

The members of this smal l phylum

range from spherical to ribbonlike andare known as comb jellies or sea wal-nuts. Traditionally, the roughly 90 ma-rine species of ctenophores (phylumCtenophora) were considered closelyrelated to the cnidarians. However,ctenophores are structurally more com-plex than cnidarians. They have analpores, so that water and other sub-stances pass completely through the an-imal. Comb jellies, abundant in theopen ocean, are transparent and usuallyonly a few centimeters long. The mem-bers of one group have two long, re-tractable tentacles that they use to cap-ture their prey.

Ctenophores propel themselvesthrough water with eight comblikeplates of fused cilia that beat in a coor-

dinated fashion (figure 44.14). They arethe largest animals that use cilia for lo-comotion. Many ctenophores are biolu-minescent, giving off bright flashes oflight particularly evident in the openocean at night.

Cnidarians and ctenophores havetissues and radial symmetry.Cnidarians have a specialized kindof cell called a cnidocyte.Ctenophores propel themselvesthrough the water by means ofeight comblike plates of fused cilia.


FIGURE 44.12Class Cubozoa. Box jelly, Chironex fleckeri.

FIGURE 44.13

Class Anthozoa.The sessile soft-bodiedsea anemone.

FIGURE 44.14A comb jelly(phylum Ctenophora).Note the comblike plates along the ridgesof the base.


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Eumetazoa: TheBilaterian Acoelomates

The Bilateria are characterized bythe second key transition in the ani-mal body plan, bilateral symmetry,which al lowed animals to achievehigh levels of specialization withinparts of their bodies. The simplestbilaterians are the acoelomates; theylack any internal cavity other thanthe digestive tract. As discussed ear-

lier, all bilaterians have three em-bryonic layers during development:ectoderm, endoderm, and meso-derm. We will focus our discussionof the acoelomates on the largestphylum of the group, the flatworms.

Phylum Platyhelminthes: TheFlatworms

Phylum Platyhelminthes consists of some 20,000 species.These ribbon-shaped, soft-bodied animals are flatteneddorsoventrally, from top to bottom. Flatworms are amongthe simplest of bilaterally symmetrical animals, but they dohave a definite head at the anterior end and they do possessorgans. Their bodies are solid: the only internal space con-sists of the digestive cavity (figure 44.15).

Flatworms range in size from a millimeter or less tomany meters long, as in some tapeworms. Most species of

flatworms are parasitic, occurring within the bodies of

many other kinds of animals (figure44.16). Other flatworms are free-liv-ing, occurring in a wide variety of

marine and freshwater habitats, aswell as moist places on land. Free-living flatworms are carnivores andscavengers; they eat various small an-imals and bits of organic debris.They move from place to place bymeans of ciliated epithelial cells,which are particularly concentratedon their ventral surfaces.

Those flatworms that have a diges-tive cavity have an incomplete gut,one with only one opening. As a re-sult, they cannot feed, digest, andeliminate undigested particles of foodsimultaneously, and thus, flatwormscannot feed continuously, as more ad-vanced animals can. Muscular con-tractions in the upper end of the gut

cause a strong sucking force allowing flatworms to ingest

their food and tear it into small bits. The gut is branchedand extends throughout the body, functioning in both di-gestion and transport of food. Cells that line the gut engulfmost of the food particles by phagocytosis and digestthem; but, as in the cnidarians, some of these particles arepartly digested extracellularly. Tapeworms, which are par-asitic flatworms, lack digestive systems. They absorb theirfood directly through their body walls.


44.3 Acoelomates are solid worms that lack a body cavity.

Sponges

Cnidarians

Flatworms

Nematodes

Mollusks

Annelids

Arthropods

Echinoderms

Chordates

Eyespot

Openingto pharynx

Protruding

pharynx

Intestinal diverticulum

Intestine

TestisEpidermis

Parenchymal muscle

Longitudinal muscles

Intestine

Circular muscles

OviductNerve cord Sperm duct

FIGURE 44.15Architecture of a solid worm.This organism is Dugesia, the familiarfreshwater planaria of many biology laboratories.


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Unlike cnidarians, flatworms have an excretory system,which consists of a network of fine tubules (little tubes) thatruns throughout the body. Cilia line the hollow centers ofbulblike flame cells located on the side branches of thetubules (see figure 58.9). Cilia in the flame cells move waterand excretory substances into the tubules and then to exitpores located between the epidermal cells. Flame cells werenamed because of the flickering movements of the tuft of

cilia within them. They primarily regulate the water bal-ance of the organism. The excretory function of flame cellsappears to be a secondary one. A large proportion of themetabolic wastes excreted by flatworms diffuses directlyinto the gut and is eliminated through the mouth.

Like sponges, cnidarians, and ctenophorans, flatwormslack circulatory systems for the transport of oxygen andfood molecules. Consequently, all flatworm cells must bewithin diffusion distance of oxygen and food. Flatworms

have thin bodies and highly branched digestive cavities thatmake such a relationship possible.The nervous system of flatworms is very simple. Like

cnidarians, some primitive flatworms have only a nerve

net. However, most members of this phylum have longi-tudinal nerve cords that constitute a simple central ner-vous system.

Free-living members of this phylum have eyespots ontheir heads. These are inverted, pigmented cups containinglight-sensitive cells connected to the nervous system. Theseeyespots enable the worms to distinguish light from dark;worms move away from strong light.

The reproductive systems of flatworms are complex.Most flatworms are hermaphroditic,with each individualcontaining both male and female sexual structures. In manyof them, fertilization is internal. When they mate, eachpartner deposits sperm in the copulatory sac of the other.The sperm travel along special tubes to reach the eggs. Inmost free-living flatworms, fertilized eggs are laid in co-coons strung in ribbons and hatch into miniature adults. Insome parasitic flatworms, there is a complex succession of

distinct larval forms. Flatworms are also capable of asexualregeneration. In some genera, when a single individual isdivided into two or more parts, each part can regenerate anentirely new flatworm.


Scolex

Scolex attached

to intestinal wall

Repeated proglottid

segments

Uterus

Hooks

Sucker

Solid worms arebilaterally symmetrical

acoelomates. Theirbodies are composedof solid layers of tissuessurrounding a centralgut. The body of manyflatworms is soft andflattened, like a piece oftape or ribbon.

Each proglottid segmentcontains reproductive organs.When segments of a wormpass out of humans in feces,embryos may be ingested bycattle or another human,transmitting the parasiteto a new host. Embryos ofthe tapeworms are releasedthrough a single genital pore

on each proglottid segment.

Genitalpore

Tapeworms are parasites thatattach by their heads to the

intestinal wall of a hostorganism. The body of amature tapeworm may reach10 meters in lengthlongerthan a truck.

PHYLUM PLATYHELMINTHES: Bilateral symmetry

Most solid wormshave a highly branchedgut that brings foodnear all tissues forabsorption directly

across the body wall.Tapeworms are a specialcase in that they havesolid bodies that lack adigestive cavity.

Proglottid

FIGURE 44.16The evolution of bilateral symmetry.Acoelomate solid worms like this beef tapeworm, Taenia saginata, are bilaterally symmetrical. Inaddition, all bilaterians have three embryonic layers and exhibit cephalization.


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Class Turbellaria: Turbellarians.Only one of the three classes of flat-wo rms , th e tu rb el la ria ns (c la ssTurbellaria) are free-living. One ofthe most familiar is the freshwatergenus Dugesia, the common planaria

used in biology laboratory exercises.Other members of this class arewidespread and often abundant inlakes, ponds, and the sea. Some alsooccur in moist places on land.

Class Trematoda: The Flukes.Two classes of parasitic flatwormslive within the bodies of other ani-

mals: flukes (class Trematoda) andtapeworms (class Cestoda). Bothgroups of worms have epithelial lay-ers resistant to the digestive enzymesand immune defenses produced bytheir hostsan important feature intheir parasitic way of life. However,they lack certain features of the free-living flatworms, such as cilia in theadult stage, eyespots, and other sen-

sory organs that lack adaptive signif-icance for an organism that liveswithin the body of another animal.

Flukes take in food through their mouth, just like theirfree-living relatives. There are more than 10,000 namedspecies, ranging in length from less than 1 millimeter tomore than 8 centimeters. Flukes attach themselves withinthe bodies of their hosts by means of suckers, anchors, orhooks. Some have a life cycle that involves only one host,

usually a fish. Most have life cycles involving two or morehosts. Their larvae almost always occur in snails, and theremay be other intermediate hosts. The final host of theseflukes is almost always a vertebrate.

To human beings, one of the most important flat-worms is the human liver fluke, Clonorchis sinensis. It livesin the bile passages of the liver of humans, cats, dogs, andpigs. It is especially common in Asia. The worms are 1 to2 centimeters long and have a complex life cycle. Al-though they are hermaphroditic, cross-fertilization usu-

ally occurs between different individuals. Eggs, each con-taining a complete, ciliated first-stage larva, ormiracidium, are passed in the feces (figure 44.17). If theyreach water, they may be ingested by a snail. Within thesnail an egg transforms into a sporocysta baglike struc-ture with embryonic germ cells. Within the sporocysts areproduced rediae,which are elongated, nonciliated larvae.These larvae continue growing within the snail, givingrise to several individuals of the tadpole-like next larval

stage, cercariae.Cercariae escape into the water, where they swim aboutfreely. If they encounter a fish of the family Cyprinidae

the family that includes carp and goldfishthey bore intothe muscles or under the scales, lose their tails, and trans-form into metacercariaewithin cysts in the muscle tissue.If a human being or other mammal eats raw infected fish,the cysts dissolve in the intestine, and the young flukes mi-grate to the bile duct, where they mature. An individual

fluke may live for 15 to 30 years in the liver. In humans, aheavy infestation of liver flukes may cause cirrhosis of theliver and death.

Other very important flukes are the blood flukes ofgenus Schistosoma.They afflict about 1 in 20 of the worldspopulation, more than 200 million people throughout trop-ical Asia, Africa, Latin America, and the Middle East.Three species ofSchistosoma cause the disease called schis-tosomiasis, or bilharzia. Some 800,000 people die each yearfrom this disease.

Recently, there has been a great deal of effort to controlschistosomiasis. The worms protect themselves in partfrom the bodys immune system by coating themselves witha variety of the hosts own antigens that effectively renderthe worm immunologically invisible (see chapter 57). De-spite this difficulty, the search is on for a vaccine thatwould cause the host to develop antibodies to one of theantigens of the young worms before they protect them-selves with host antigens. This vaccine would prevent hu-

mans from infection. The disease can be cured with drugsafter infection.


Adult fluke

Egg containingmiracidium

Miracidium hatchesafter being eatenby snail

SporocystRedia

Cercaria

Metacercarialcysts in fishmuscle Bile duct

Liver

Raw, infected fishis consumed byhumans or othermammals

FIGURE 44.17

Life cycle of the human liver fluke, Clonorchis sinensis.


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Class Cestoda: The Tapeworms. Class Cestoda is thethird class of flatworms; like flukes, they live as parasiteswithin the bodies of other animals. In contrast to flukes,tapeworms simply hang on to the inner walls of their hostsby means of specialized terminal attachment organs and ab-sorb food through their skins. Tapeworms lack digestive

cavities as well as digestive enzymes. They are extremelyspecialized in relation to their parasitic way of life. Mostspecies of tapeworms occur in the intestines of vertebrates,about a dozen of them regularly in humans.

The long, flat bodies of tapeworms are divided intothree zones: the scolex, or attachment organ; the unseg-mented neck; and a series of repetitive segments, theproglottids (see figure 44.16). The scolex usually bearsseveral suckers and may also have hooks. Each proglottid is

a complete hermaphroditic unit, containing both male andfemale reproductive organs. Proglottids are formed contin-uously in an actively growing zone at the base of the neck,with maturing ones moving farther back as new ones areformed in front of them. Ultimately the proglottids nearthe end of the body form mature eggs. As these eggs arefertilized, the zygotes in the very last segments begin to dif-ferentiate, and these segments fill with embryos, break off,and leave their host with the hosts feces. Embryos, eachsurrounded by a shell, emerge from the proglottid through

a pore or the ruptured body wall. They are deposited onleaves, in water, or in other places where they may bepicked up by another animal.

The beef tapeworm Taenia saginata occurs as a juvenilein the intermuscular tissue of cattle but as an adult in theintestines of human beings. A mature adult beef tapewormmay reach a length of 10 meters or more. These worms at-tach themselves to the intestinal wall of their host by ascolex with four suckers. The segments that are shed from

the end of the worm pass from the human in the feces and

may crawl onto vegetation. The segments ultimately rup-ture and scatter the embryos. Embryos may remain viablefor up to five months. If they are ingested by cattle, theyburrow through the wall of the intestine and ultimatelyreach muscle tissues through the blood or lymph vessels.About 1% of the cattle in the United States are infected,

and some 20% of the beef consumed is not federally in-spected. When infected beef is eaten rare, infection of hu-mans by these tapeworms is likely. As a result, the beeftapeworm is a frequent parasite of humans.

Phylum Nemertea: The Ribbon Worms

The phylogenetic relationship of phylum Nemertea (figure44.18) to other free-living flatworms is unclear. Ne-

merteans are often called ribbon worms or proboscisworms. These aquatic worms have the body plan of a flat-worm, but also possess a fluid-filled sac that may be a prim-itive coelom. This sac serves as a hydraulic power sourcefor their proboscis, a long muscular tube that can be thrustout quickly from a sheath to capture prey. Shaped like athread or a ribbon, ribbon worms are mostly marine andconsist of about 900 species. Ribbon worms are large, often10 to 20 centimeters and sometimes many meters in length.They are the simplest animals that possess a complete di-

gestive system, one that has two separate openings, amouth and an anus. Ribbon worms also exhibit a circula-tory system in which blood flows in vessels. Many impor-tant evolutionary trends that become fully developed inmore advanced animals make their first appearance in theNemertea.

The acoelomates, typified by flatworms, are the mostprimitive bilaterally symmetrical animals and thesimplest animals in which organs occur.


FIGURE 44.18

A ribbon worm, Lineus(phylumNemertea). This is the simplest animalwith a complete digestive system.


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The Pseudocoelomates

Al l bi la te ri an s except so li d wo rms

possess an internal body cavity, thethird key transition in the animalbody plan. Seven phyla are character-ized by their possession of a pseudo-coel (see figure 44.4). Their evolu-tionary relationships remain unclear,with the possibility that the pseudo-coelomate condition arose indepen-dently many times. The pseudocoelserves as a hydrostatic skeletonone

that gains its rigidity from being filledwith fluid under pressure. The ani-mals muscles can work against thisskeleton, thus making the move-ment of pseudocoelomates far moreefficient than that of the acoelomates.

Pseudocoelomates lack a definedcirculatory system; this role is per-formed by the fluids that move within

the pseudocoel. Most pseudocoelo-mates have a complete, one-way di-gestive tract that acts like an assemblyline. Food is first broken down, thenabsorbed, and then treated andstored.

Phylum Nematoda:The Roundworms

Nematodes, eelworms, and otherroundworms constitute a large phy-lum, Nematoda, with some 12,000recognized species. Scientists estimatethat the actual number might ap-proach 100 times that many. Membersof this phylum are found everywhere.Nematodes are abundant and diversein marine and freshwater habitats, and

many members of this phylum are parasites of animals(figure 44.19) and plants. Many nematodes are micro-scopic and live in soil. It has been estimated that a spade-ful of fertile soil may contain, on the average, a millionnematodes.

Nematodes are bilaterally symmetrical, unsegmentedworms. They are covered by a flexible, thick cuticle,which is molted as they grow. Their muscles constitute alayer beneath the epidermis and extend along the lengthof the worm, rather than encircling its body. These lon-gitudinal muscles pull both against the cuticle and thepseudocoel, which forms a hydrostatic skeleton. When

nematodes move, their bodies whipabout from side to side.

Near the mouth of a nematode, at

its anterior end, are usually 16 raised,hairlike, sensory organs. The mouth isoften equipped with piercing organscalled stylets. Food passes throughthe mouth as a result of the suckingaction of a muscular chamber calledthe pharynx. After passing through ashort corridor into the pharynx, foodcontinues through the other portionsof the digestive tract, where it is bro-

ken down and then digested. Some ofthe water with which the food hasbeen mixed is reabsorbed near the endof the digestive tract, and material thathas not been digested is eliminatedthrough the anus (figure 44.20).

Nematodes completely lack flagellaor cilia, even on sperm cells. Repro-

duction in nematodes is sexual, with

sexes usually separate. Their develop-ment is simple, and the adults consist ofvery few cells. For this reason, nema-todes have become extremely importantsubjects for genetic and developmentalstudies (see chapter 17). The 1-mil-limeter-long Caenorhabditis elegansma-tures in only three days, its body istransparent, and it has only 959 cells. Itis the only animal whose complete de-

velopmental cellular anatomy is known.About 50 species of nematodes, in-

cluding several that are rather com-mon in the United States, regularlyparasitize human beings. The most se-rious common nematode-caused dis-ease in temperate regions is trichi-nosis, caused by worms of the genusTrichinella. These worms live in the

small intestine of pigs, where fertilized female wormsburrow into the intestinal wall. Once it has penetratedthese tissues, each female produces about 1500 liveyoung. The young enter the lymph channels and travel tomuscle tissue throughout the body, where they matureand form highly resistant, calcified cysts. Infection inhuman beings or other animals arises from eating under-cooked or raw pork in which the cysts of Trichinella arepresent. If the worms are abundant, a fatal infection canresult, but such infections are rare; only about 20 deathsin the United States have been attributed to trichinosisduring the past decade.



Sponges

Cnidarians

Flatworms

Nematodes

Mollusks

Annelids

Arthropods

Echinoderms

Chordates

FIGURE 44.19Trichinella nematode encysted in pork.The serious disease trichinosis can resultfrom eating undercooked pork or bear meatcontaining such cysts.


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Phylum Rotifera: Rotifers

Phylum Rotifera includes common, small, bilaterally sym-metrical, basically aquatic animals that have a crown ofcilia at their heads. Rotifers are pseudocoelomates but arevery unlike nematodes. They have several features thatsuggest their ancestors may have resembled flatworms.There are about 2000 species of this phylum. While a few

rotifers live in soil or in the capillary water in cushions ofmosses, most occur in fresh water, and they are commoneverywhere. Very few rotifers are marine. Most rotifersare between 50 and 500 micrometers in length, smallerthan many protists.

Rotifers have a well-developed food-processing appara-tus. A conspicuous organ on the tip of the head called thecorona gathers food. It is composed of a circle of ciliawhich sweeps their food into their mouths , as well asbeing used for locomotion. Rotifers are often calledwheel animals because the cilia, when they are beatingtogether, resemble the movement of spokes radiatingfrom a wheel.

A Relatively New Phylum: Cycliophora

In December 1995, two Danish biologists reported the dis-covery of a strange new kind of creature, smaller than a pe-riod on a printed page. The tiny organism had a strikingcircular mouth surrounded by a ring of fine, hairlike ciliaand has so unusual a life cycle that they assigned it to anentirely new phylum, Cycliophora (Greek for carrying a

small wheel). There are only about 35 known animalphyla, so finding a new one is extremely rare! When thelobster to which it is attached starts to molt, the tiny sym-biont begins a bizarre form of sexual reproduction. Dwarfmales emerge, with nothing but brains and reproductiveorgans. Each dwarf male seeks out another female sym-biont on the molting lobster and fertilizes its eggs, generat-ing free-swimming individuals that can seek out anotherlobster and renew the life cycle.

The pseudocoelomates, including nematodes androtifers, all have fluid-filled pseudocoels.


The pseudocoel of a nematodeseparates the endoderm-lined gut fromthe rest of the body. The digestive tractis one-way: food enters the mouth atone end of the worm and leavesthrough the anus at the other end.

The nematode's body is coveredwith a flexible, thick cuticle that is shedas the worm grows. Muscles extend

along the length of the body ratherthan encircling it, which allows theworm to flex its body to move throughthe soil.

Roundworms are bilaterallysymmetrical, cylindrical, unsegmented

worms. Most nematodes are verysmall, less than a millimeter longhundreds of thousands may live in ahandful of fertile soil.

An adult nematode consists of veryfew cells. Caenorhabditis eleganshas

exactly 959 cells and is the only animalwhose complete cellular anatomy isknown.

Pseudocoel

PHYLUM NEMATODA: Body cavity

Nematodes have excretory ducts thatpermit them to conserve water and liveon land. Other roundworms possessexcretory cells called flame cells.Intestine

Intestine

Oviduct

Cuticle

Nerve cord

Ovary

Ovary

Muscle

Uterus

Uterus

Excretory duct

Anus

Genital pore

Pharynx

Mouth

Excretory pore

FIGURE 44.20The evolution of a simple body cavity.The major innovation in body design in roundworms (phylum Nematoda) is a body cavitybetween the gut and the body wall. This cavity is the pseudocoel. It allows chemicals to circulate throughout the body and prevents organsfrom being deformed by muscle movements.


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Reevaluating How the Animal Body

Plan EvolvedThe great diversity see in the body plan of animals is diffi-cult to fit into any one taxonomic scheme. Biologists havetraditionally inferred the general relationships among the35 animal phyla by examining what seemed to be funda-mental characterssegmentation, possession of a coelom,and so on. The general idea has been that such charactersare most likely to be conserved during a groups evolution.Animal phyla that share a fundamental character are more

likely to be closely related to each other than to other phylathat do not exhibit the character. The phylogeny presentedin figure 44.2 is a good example of the sort of taxonomythis approach has generated.

However, not every animal can be easily accommodatedby this approach. Take, for example, the myzostomids (fig-ure 44.21), an enigmatic and anatomically bizarre group ofmarine animals that are parasites or symbionts of echino-derms. Myzostomid fossils are found associated with echin-oderms since the Ordovician, so the myzostomid-echino-

derm relationship is a very ancient one. Their long historyof obligate association has led to the loss or simplificationof many myzostomid body elements, leaving them, for ex-ample, with no body cavity (they are acoelomates) and onlyincomplete segmentation.

This character loss has led to considerable disagreementamong taxonomists. However, while taxonomists have dis-agreed about the details, all have generally allied myzosto-mids is some fashion with the annelids, sometimes within

the polychaetes, sometimes as a separate phylum closely al-lied to the annelids.Recently, this view has been challenged. New taxonomi-

cal comparisons using molecular data have come to verydifferent conclusions. Researchers examined two compo-nents of the protein synthesis machinery, the small riboso-mal subunit rRNA gene, and an elongation factor gene(called 1 alpha). The phylogeny they obtain does not placethe myzostomids in with the annelids. Indeed, they findthat the myzostomids have no close links to the annelids at

all. Instead, surprisingly, they are more closely allied withthe flatworms!

This result hints strongly that the key morphologicalcharacters that biologists have traditionally used to con-struct animal phylogeniessegmentation, coeloms,jointed appendages, and the likeare not the conservativecharacters we had supposed. Among the myzostomidsthese features appear to have been gained and lost againduring the course of their evolution. If this unconservative

evolutionary pattern should prove general, our view of theevolution of the animal body plan, and how the various an-imal phyla relate to one another, will soon be in need ofmajor revision.

Molecular Phylogenies

The last decade has seen a wealth of new molecular se-

quence data on the various animal groups. The animal phy-logenies that these data suggest are often significantly atodds with the traditional phylogeny used in this text andpresented in figure 44.2. One such phylogeny, developedfrom ribosomal RNA studies, is presented in figure 44.22.It is only a rough outline; in the future more data shouldallow us to resolve relationships within groupings. Still, it isclear that major groups are related in very different ways inthe molecular phylogeny than in the more traditional one.

At present, molecular phylogenetic analysis of the ani-

mal kingdom is in its infancy. Molecular phylogenies devel-oped from different molecules often tend to suggest differ-ent evolutionary relationships. However, the childhood ofthis approach is likely to be short. Over the next few years,a mountain of additional molecular data can be anticipated.As more data are brought to bear, we can hope that theconfusion will lessen, and that a consensus phylogeny willemerge. When and if this happens, it is likely to be verydifferent from the traditional view.

The use of molecular data to construct phylogenies islikely to significantly alter our understanding ofrelationships among the animal phyla.


44.5 The coming revolution in animal taxonomy will likely alter traditionalphylogenies.

FIGURE 44.21A taxonomic puzzle. Myzostoma martensenihas no body cavity

and incomplete segmentation. Animals such as this present aclassification challenge, causing taxonomists to reconsidertraditional animal phylogenies based on fundamental characters.


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Vertebrates

Cephalochordates

Urochordates

Hemichordates

Echinoderms

Brachiopods

Bryozoans

Phoronids

Sipunculans

Mollusks

Echiurians

Pogonophorans

Annelids

Onychophorans

Tardigrades

Arthropods

Gnathostomulids

Rotifers

Gastrotrichs

Nematodes

Priapulids

Kinorhynchs

Platyhelminthes

Nemerteans

Entoprocts

Ctenophorans

Cnidarians

Poriferans Poriferans

Cnidarians

Ctenophorans

Gastrotrichs

Nematodes

Priapulids

Kinorhynchs

Onychophorans

Tardigrades

Arthropods

Bryozoans

Entoprocts

Platyhelminthes

Pogonophorans

Brachiopods

Phoronids

Nemerteans

Annelids

Echiurans

Mollusks

Sipunculans

Gnathostomulids

Rotifers

Vertebrates

Cephalochordates

Urochordates

Hemichordates

Echinoderms

Bilateria

Bilateria

Deuterostome

s

Deuterostome

s

Protostomes

P

rotostomes

Lophophorates

Ecdysozoans

Radiata

Acoelomates

Pseudocoelomates

Coe

lomates

Lophotrochozoans

(a) Traditional phylogeny (b) Molecular phylogeny

FIGURE 44.22

Traditional versus molecular animal phylogenies. (a) Traditional phylogenies are based on fundamental morphological characters.(After L. H. Hyman, The Invertebrates, 1940.) (b) More recent phylogenies are often based on molecular analyses, this one on comparisonsof rRNA sequence differences among the animal phyla. (After Adoutte, et al.,Proc. Nat. Acad. Sci97: p. 4454, 2000.)

Ch 44


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Chapter 44Summary Questions Media Resources

44.1 Animals are multicellular heterotrophs without cell walls.

Animals are heterotrophic, multicellular, and usuallyhave the ability to move. Almost all animalsreproduce sexually. Animal cells lack rigid cell wallsand digest their food internally.

The kingdom Animalia is divided into twosubkingdoms: Parazoa, which includes only theasymmetrical phylum Porifera, and Eumetazoa,characterized by body symmetry.

1.What are the characteristicsthat distinguish animals fromother living organisms?

2.What are the twosubkingdoms of animals? Howdo they differ in terms ofsymmetry and bodyorganization?

The sponges (phylum Porifera) are characterized byspecialized, flagellated cells called choanocytes. Theydo not possess tissues or organs, and most species lacksymmetry in their body organization.

Cnidarians (phylum Cnidaria) are predominantlymarine animals with unique stinging cells calledcnidocytes, each of which contains a specialized

harpoonlike apparatus, or nematocyst.

3. From what kind of ancestordid sponges probably evolve?

4.What are the specializedcells used by a sponge to capturefood?

5.What are the two wayssponges reproduce? What dolarval sponges look like?

6.What is a planula?

44.2 The simplest animals are not bilaterally symmetrical.

Acoelomates lack an internal cavity, except for thedigestive system, and are the simplest animals thathave organs.

The most prominent phylum of acoelomates,

Platyhelminthes, includes the free-living flatwormsand the parasitic flukes and tapeworms.

Ribbon worms (phylum Nemertea) are similar tofree-living flatworms, but have a complete digestivesystem and a circulatory system in which the bloodflows in vessels.

7.What body plan do membersof the phylum Platyhelminthespossess? Are these animalsparasitic or free-living? How dothey move from place to place?

8. How are tapewormsdifferent from flukes? How dotapeworms reproduce?

44.3 Acoelomates are solid worms that lack a body cavity.

Pseudocoelomates, exemplified by the nematodes(phylum Nematoda), have a body cavity that developsbetween the mesoderm and the endoderm.

Rotifers (phylum Rotifera), or wheel animals, are verysmall freshwater pseudocoelomates.

9.Why are nematodesstructurally unique in the animalworld?

10. How do rotifers capturefood?


Molecular data are suggesting animal phylogeniesthat are in considerable disagreement with traditionalphylogenies.

11.With what group aremyzostomids most closely allied?

44.5 The coming revolution in animal taxonomy will likely altertraditional phylogenies.

www.mhhe.com/raven6e www.biocourse.com

Activity: Invertebrates Characteristics of

Invertebrates Body Organization

Symmetry in Nature Posterior to Anterior Sagittal Plane Frontal to Coronal

Plane Transvere/Cross-

sectional Planes

Sponges Radical Phyla

Bilateral Acoelomates

Student Research:Parasitic Flatworms

Pseudocoelomates

Student Research:Molecular Phylogenyof Gastropods

Documents

Animal Development Chapter 44