mollusks and annelids - biolympiads · mollusks are the second-largest phylum of animals in terms...

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45Mollusks and

Annelids

Concept Outline

45.1 Mollusks were among the first coelomates.

Coelomates. Next to the arthropods, mollusks comprisethe second most diverse phylum and include snails, clams,and octopuses. There are more terrestrial mollusk speciesthan terrestrial vertebrates!Body Plan of the Mollusks. The mollusk body plan ischaracterized by three distinct sections, a unique raspingtongue, and distinctive free-swimming larvae also found inannelid worms.The Classes of Mollusks. The three major classes ofmollusks are the gastropods (snails and slugs), the bivalves(oysters and clams), and the cephalopods (octopuses andsquids). While they seem very different at first glance, oncloser inspection, they all have the same basic mollusk bodyplan.

45.2 Annelids were the first segmented animals.

Segmented Animals. Annelids are segmented coelomateworms, most of which live in the sea. The annelid body iscomposed of numerous similar segments.Classes of Annelids. The three major classes of annelidsare the polychaetes (marine worms), the oligochaetes(earthworms and related freshwater worms), and thehirudines (leeches).

45.3 Lophophorates appear to be a transitional group.

Lophophorates. The three phyla of lophophorates sharea unique ciliated feeding structure, but differ in many otherways. Although acoelomates and pseudocoelomates have proven

very successful, a third way of organizing the animalbody has also evolved, one that occurs in the bulk of the ani-mal kingdom. We will begin our discussion of the coelomateanimals with mollusks, which include such animals as clams,snails, slugs, and octopuses. Annelids (figure 45.1), such asearthworms, leeches, and seaworms, are also coelomates, butin addition, were the earliest group of animals to evolve seg-mented bodies. The lophophorates, a group of marine ani-mals united by a distinctive feeding structure called thelophophore, have features intermediate between those of pro-tostomes and deuterostomes and will also be discussed in thischapter. The remaining groups of coelomate animals will bediscussed in chapters 46, 47, and 48.

FIGURE 45.1An annelid, the Christmas-tree worm, Spirobranchusgiganteus. Mollusks and annelids inhabit both terrestrial andaquatic habitats. They are large and successful groups, with someof their most spectacular members represented in marineenvironments.

As a group, mollusks are an impor-tant source of food for humans. Oys-ters, clams, scallops, mussels, octopuses,and squids are among the culinary deli-cacies that belong to this large phylum.Mollusks are also of economic signifi-cance to us in many other ways. For ex-ample, pearls are produced in oysters,and the material called mother-of-pearl, often used in jewelry and otherdecorative objects, is produced in theshells of a number of different mol-lusks, but most notably in the snailcalled abalone. Mollusks are not whollybeneficial to humans, however. Bivalvemollusks called shipworms burrowthrough wood submerged in the sea,damaging boats, docks, and pilings.The zebra mussel has recently invadedNorth American ecosystems from Eu-rope via the ballast water of cargo shipsfrom Europe, wreaking havoc in manyaquatic ecosystems. Slugs and terres-trial snails often cause extensive dam-age to garden flowers, vegetables, andcrops. Other mollusks serve as hosts tothe intermediate stages of many seriousparasites, including several nematodesand flatworms, which we discussed inchapter 44.

Mollusks range in size from almostmicroscopic to huge, although mostmeasure a few centimeters in theirlargest dimension. Some, however, areminute, while others reach formidablesizes. The giant squid, which is occa-sionally cast ashore but has rarely beenobserved in its natural environment,may grow up to 21 meters long!Weighing up to 250 kilograms, thegiant squid is the largest invertebrate

and, along with the giant clam (figure 45.4), the heaviest.Millions of giant squid probably inhabit the deep regions ofthe ocean, even though they are seldom caught. Anotherlarge mollusk is the bivalve Tridacna maxima, the giantclam, which may be as long as 1.5 meters and may weigh asmuch as 270 kilograms.

Mollusks are the second-largest phylum of animals interms of named species; mollusks exhibit a variety ofbody forms and live in many different environments.

900 Part XII Animal Diversity

CoelomatesThe evolution of the coelom was a sig-nificant advance in the structure of theanimal body. Coelomates have a newbody design that repositions the fluidand allows the development of com-plex tissues and organs. This new bodyplan also made it possible for animalsto evolve a wide variety of differentbody architectures and to grow tomuch larger sizes than acoelomate ani-mals. Among the earliest groups ofcoelomates were the mollusks and theannelids.

Mollusks

Mollusks (phylum Mollusca) are an ex-tremely diverse animal phylum, secondonly to the arthropods, with over110,000 described species. Mollusksinclude snails, slugs, clams, scallops,oysters, cuttlefish, octopuses, andmany other familiar animals (figure45.2). The durable shells of some mol-lusks are often beautiful and elegant;they have long been favorite objectsfor professional scientists and amateursalike to collect, preserve, and study.Chitons and nudibranchs are less fa-miliar marine mollusks. Mollusks arecharacterized by a coelom, and whilethere is extraordinary diversity in thisphylum, many of the basic compo-nents of the mollusk body plan can beseen in figure 45.3.

Mollusks evolved in the oceans, andmost groups have remained there. Ma-rine mollusks are widespread andoften abundant. Some groups of mol-lusks have invaded freshwater and terrestrial habitats, in-cluding the snails and slugs that live in your garden. Ter-restrial mollusks are often abundant in places that are atleast seasonally moist. Some of these places, such as thecrevices of desert rocks, may appear very dry, but eventhese habitats have at least a temporary supply of water atcertain times. There are so many terrestrial mollusks thatonly the arthropods have more species adapted to a ter-restrial way of life. The 35,000 species of terrestrial mol-lusks far outnumber the roughly 20,000 species of terres-trial vertebrates.


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FIGURE 45.2A mollusk. The blue-ringed octopus is oneof the few mollusks dangerous to humans.Strikingly beautiful, it is equipped with asharp beak and poison glands—divers giveit a wide berth!

Chapter 45 Mollusks and Annelids 901

PHYLUM MOLLUSCA: Coelom

Radula

Mantle

Digestivegland

Stomach

GonadIntestine

HeartCoelom

Nephridium

Shell

Gill

Mantle cavity

Anus

Retractor musclesFootNerve collar

Mouth

The mantle is a heavy fold of tissuewrapped around the mollusk body like acape. The cavity between the mantle andthe body contains gills, which captureoxygen from the water passing throughthe mantle cavity. In some mollusks, likesnails, the mantle secretes a hardouter shell.

Many mollusks are carnivores. They locateprey using chemosensory structures. Withinthe mouth of a snail are horny jaws and a unique rasping tongue called a radula.

Snails creep along the ground on a muscular foot. Squid can shoot through the water by squeezing water out of themantle cavity, in a kind of jet propulsion.

Mollusks were among the first animals to develop an efficient excretory system. Tubular structures called nephridia gather wastes from the coelom and dischargethem into the mantle cavity.

Snails have a three-chambered heartand an open circulation system. Thecoelom is confined to a small cavityaround the heart.

FIGURE 45.3Evolution of the coelom. A generalized mollusk body plan is shown above. The body cavity of a mollusk is a coelom, which is completelyenclosed within the mesoderm. This allows physical contact between the mesoderm and the endoderm, permitting interactions that lead todevelopment of highly specialized organs such as a stomach.

FIGURE 45.4Giant clam. Second only to thearthropods in number of described species,members of the phylum Mollusca occupyalmost every habitat on earth. This giantclam, Tridacna maxima, has a green colorthat is caused by the presence of symbioticdinoflagellates (zooxanthellae). Throughphotosynthesis, the dinoflagellatesprobably contribute most of the foodsupply of the clam, although it remains afilter feeder like most bivalves. Someindividual giant clams may be nearly 1.5meters long and weigh up to 270kilograms.

Body Plan of the MollusksIn their basic body plan (figure 45.5), mollusks have dis-tinct bilateral symmetry. Their digestive, excretory, andreproductive organs are concentrated in a visceral mass,and a muscular foot is their primary mechanism of loco-motion. They may also have a differentiated head at theanterior end of the body. Folds (often two) arise from thedorsal body wall and enclose a cavity between themselvesand the visceral mass; these folds constitute the mantle. Insome mollusks the mantle cavity acts as a lung; in others itcontains gills. Gills are specialized portions of the mantlethat usually consist of a system of filamentous projectionsrich in blood vessels. These projections greatly increasethe surface area available for gas exchange and, therefore,the animal’s overall respiratory potential. Mollusk gills arevery efficient, and many gilled mollusks extract 50% ormore of the dissolved oxygen from the water that passesthrough the mantle cavity. Finally, in most members ofthis phylum, the outer surface of the mantle also secretes aprotective shell.

A mollusk shell consists of a horny outer layer, rich inprotein, which protects the two underlying calcium-richlayers from erosion. The middle layer consists of denselypacked crystals of calcium carbonate. The inner layer ispearly and increases in thickness throughout the animal’slife. When it reaches a sufficient thickness, this layer isused as mother-of-pearl. Pearls themselves are formedwhen a foreign object, like a grain of sand, becomes lodgedbetween the mantle and the inner shell layer of bivalvemollusks (two-shelled), including clams and oysters. Themantle coats the foreign object with layer upon layer ofshell material to reduce irritation caused by the object.The shell of mollusks serves primarily for protection.Many species can withdraw for protection into their shellif they have one.

In aquatic mollusks, a continuous stream of water passesinto and out of the mantle cavity, drawn by the cilia on thegills. This water brings in oxygen and, in the case of the bi-valves, also brings in food; it also carries out waste materi-als. When the gametes are being produced, they are fre-quently carried out in the same stream.

The foot of a mollusk is muscular and may be adaptedfor locomotion, attachment, food capture (in squids and oc-topuses), or various combinations of these functions. Somemollusks secrete mucus, forming a path that they glidealong on their foot. In cephalopods—squids and octo-puses—the foot is divided into arms, also called tentacles.In some pelagic forms, mollusks that are perpetually free-swimming, the foot is modified into wing-like projectionsor thin fins.

One of the most characteristic features of all the mol-lusks except the bivalves is the radula, a rasping, tongue-like organ used for feeding. The radula consists primarilyof dozens to thousands of microscopic, chitinous teetharranged in rows (figure 45.6). Gastropods (snails andtheir relatives) use their radula to scrape algae and otherfood materials off their substrates and then to convey thisfood to the digestive tract. Other gastropods are activepredators, some using a modified radula to drill throughthe shells of prey and extract the food. The small holesoften seen in oyster shells are produced by gastropodsthat have bored holes to kill the oyster and extract itsbody for food.

The circulatory system of all mollusks exceptcephalopods consists of a heart and an open system inwhich blood circulates freely. The mollusk heart usuallyhas three chambers, two that collect aerated blood from thegills, while the third pumps it to the other body tissues. Inmollusks, the coelom takes the form of a small cavityaround the heart.


Radula

Gut

Gill

GutGill

Radula

Foot

Mantle

Mantlecavity

Shell

GutGillTentacles

Foot

Mantle

Mantlecavity

Shell

Radula

Gut

GillRadula Foot

Mantle

Mantle cavity

Shell

GutGill

Foot

Mantle

Mantle cavity

Shell

Cephalopods

Chitons

Bivalves

Hypothetical Ancestor

Gastropods

FIGURE 45.5Body plans among the mollusks.

Nitrogenous wastes are removedfrom the mollusk by one or two tubu-lar structures called nephridia. A typi-cal nephridium has an open funnel, thenephrostome, which is lined withcilia. A coiled tubule runs from thenephrostome into a bladder, which inturn connects to an excretory pore.Wastes are gathered by the nephridiafrom the coelom and discharged intothe mantle cavity. The wastes are thenexpelled from the mantle cavity by thecontinuous pumping of the gills. Sug-ars, salts, water, and other materials arereabsorbed by the walls of thenephridia and returned to the animal’sbody as needed to achieve an appropri-ate osmotic balance.

In animals with a closed circulatorysystem, such as annelids, cephalopodmollusks, and vertebrates, the coiledtubule of a nephridium is surroundedby a network of capillaries. Wastes areextracted from the circulatory systemthrough these capillaries and are trans-ferred into the nephridium, then sub-sequently discharged. Salts, water, andother associated materials may also bereabsorbed from the tubule of thenephridium back into the capillaries.For this reason, the excretory systemsof these coelomates are much more ef-ficient than the flame cells of theacoelomates, which pick up substancesonly from the body fluids. Molluskswere one of the earliest evolutionarylines to develop an efficient excretorysystem. Other than chordates, coelo-mates with closed circulation have sim-ilar excretory systems.

Reproduction in Mollusks

Most mollusks have distinct male and female individuals, al-though a few bivalves and many gastropods are hermaphro-ditic. Even in hermaphroditic mollusks, cross-fertilization ismost common. Remarkably, some sea slugs and oysters areable to change from one sex to the other several times dur-ing a single season.

Most aquatic mollusks engage in external fertilization.The males and females release their gametes into the water,where they mix and fertilization occurs. Gastropods moreoften have internal fertilization, however, with the male in-serting sperm directly into the female’s body. Internal fer-tilization is one of the key adaptations that allowed gas-tropods to colonize the land.

Many marine mollusks have free-swimming larvae calledtrochophores (figure 45.7a), which closely resemble the lar-val stage of many marine annelids. Trochophores swim bymeans of a row of cilia that encircles the middle of theirbody. In most marine snails and in bivalves, a second free-swimming stage, the veliger, follows the trochophore stage.This veliger stage, has the beginnings of a foot, shell, andmantle (figure 45.7b). Trochophores and veligers drift widelyin the ocean currents, dispersing mollusks to new areas.

Mollusks were among the earliest animals to evolve anefficient excretory system. The mantle of mollusks notonly secretes their protective shell, but also forms acavity that is essential to respiration.


(a) (b)

FIGURE 45.6Structure of the radula in a snail. (a) The radula consists of chitin and is covered withrows of teeth. (b) Enlargement of the rasping teeth on a radula.

(a) (b)

FIGURE 45.7Stages in the molluscan life cycle. (a) The trochophore larva of a mollusk. Similar larvae,as you will see, are characteristic of some annelid worms as well as a few other phyla.(b) Veliger stage of a mollusk.

The Classes of MollusksThere are seven classes of mollusks. We will examine fourclasses of mollusks as representatives of the phylum: (1)Polyplacophora—chitons; (2) Gastropoda—snails, slugs,limpets, and their relatives; (3) Bivalvia—clams, oysters,scallops, and their relatives; and (4) Cephalopoda—squids,octopuses, cuttlefishes, and nautilus. By studying livingmollusks and the fossil record, some scientists have de-duced that the ancestral mollusk was probably a dorsoven-trally flattened, unsegmented, wormlike animal that glidedon its ventral surface. This animal may also have had achitinous cuticle and overlapping calcareous scales. Otherscientists believe that mollusks arose from segmented an-cestors and became unsegmented secondarily.

Class Polyplacophora: The Chitons

Chitons are marine mollusks that have oval bodies witheight overlapping calcareous plates. Underneath the plates,the body is not segmented. Chitons creep along using abroad, flat foot surrounded by a groove or mantle cavity inwhich the gills are arranged. Most chitons are grazing her-bivores that live in shallow marine habitats, but some live atdepths of more than 7000 meters.

Class Gastropoda: The Snails and Slugs

The class Gastropoda contains about 40,000 describedspecies of snails, slugs, and similar animals. This class isprimarily a marine group, but it also contains many fresh-water and terrestrial mollusks (figure 45.8). Most gas-tropods have a shell, but some, like slugs and nudibranchs,have lost their shells through the course of evolution. Gas-tropods generally creep along on a foot, which may bemodified for swimming.

The heads of most gastropods have a pair of tentacleswith eyes at the ends. These tentacles have been lost insome of the more advanced forms of the class. Within themouth cavity of many members of this class are horny jawsand a radula.

During embryological development, gastropods undergotorsion. Torsion is the process by which the mantle cavityand anus are moved from a posterior location to the frontof the body, where the mouth is located. Torsion isbrought about by a disproportionate growth of the lateralmuscles; that is, one side of the larva grows much morerapidly than the other. A 120-degree rotation of the vis-ceral mass brings the mantle cavity above the head andtwists many internal structures. In some groups of gas-tropods, varying degrees of detorsion have taken place. Thecoiling, or spiral winding, of the shell is a separate process.This process has led to the loss of the right gill and rightnephridium in most gastropods. Thus, the visceral mass ofgastropods has become bilaterally asymmetrical during thecourse of evolution.

Gastropods display extremely varied feeding habits.Some are predatory, others scrape algae off rocks (oraquarium glass), and others are scavengers. Many are herbi-vores, and some terrestrial ones are serious garden andagricultural pests. The radula of oyster drills is used to boreholes in the shells of other mollusks, through which thecontents of the prey can be removed. In cone shells, theradula has been modified into a kind of poisonous harpoon,which is shot with great speed into the prey.

Sea slugs, or nudibranchs, are active predators; a fewspecies of nudibranchs have the extraordinary ability to ex-tract the nematocysts from the cnidarian polyps they eatand transfer them through their digestive tract to the sur-face of their gills intact and use them for their own protec-tion. Nudibranchs are interesting in that they get theirname from their gills, which instead of being enclosedwithin the mantle cavity are exposed along the dorsal sur-face (nudi, “naked”; branch; “gill”).

In terrestrial gastropods, the empty mantle cavity, whichwas occupied by gills in their aquatic ancestors, is extremelyrich in blood vessels and serves as a lung, in effect. Thisstructure evolved in animals living in environments withplentiful oxygen; it absorbs oxygen from the air much moreeffectively than a gill could, but is not as effective underwater.

Class Bivalvia: The Bivalves

Members of the class Bivalvia include the clams, scallops,mussels, and oysters. Bivalves have two lateral (left andright) shells (valves) hinged together dorsally (figure 45.9).A ligament hinges the shells together and causes them togape open. Pulling against this ligament are one or twolarge adductor muscles that can draw the shells together.


FIGURE 45.8A gastropod mollusk. The terrestrial snail, Allogona townsendiana.

The mantle secretes the shells and ligament and envelopsthe internal organs within the pair of shells. The mantle isfrequently drawn out to form two siphons, one for an in-coming and one for an outgoing stream of water. Thesiphons often function as snorkels to allow bivalves to filterwater through their body while remaining almost com-pletely buried in sediments. A complex folded gill lies oneach side of the visceral mass. These gills consist of pairs offilaments that contain many blood vessels. Rhythmic beat-ing of cilia on the gills creates a pattern of water circula-tion. Most bivalves are sessile filter-feeders. They extractsmall organisms from the water that passes through theirmantle cavity.

Bivalves do not have distinct heads or radulas, differingfrom gastropods in this respect (see figure 45.5). However,most have a wedge-shaped foot that may be adapted, in dif-ferent species, for creeping, burrowing, cleansing the ani-mal, or anchoring it in its burrow. Some species of clamscan dig into sand or mud very rapidly by means of muscularcontractions of their foot.

Bivalves disperse from place to place largely as larvae.While most adults are adapted to a burrowing way of life,some genera of scallops can move swiftly through the waterby using their large adductor muscles to clap their shells to-gether. These muscles are what we usually eat as “scallops.”The edge of a scallop’s body is lined with tentacle-like pro-jections tipped with complex eyes.

There are about 10,000 species of bivalves. Most speciesare marine, although many also live in fresh water. Over500 species of pearly freshwater mussels, or naiads, occur inthe rivers and lakes of North America.

Class Cephalopoda: The Octopuses, Squids, andNautilus

The more than 600 species of the class Cephalopoda—oc-topuses, squids, and nautilus—are the most intelligent ofthe invertebrates. They are active marine predators thatswim, often swiftly, and compete successfully with fish.The foot has evolved into a series of tentacles equippedwith suction cups, adhesive structures, or hooks that seizeprey efficiently. Squids have 10 tentacles (figure 45.10); oc-topuses, as indicated by their name, have eight; and thenautilus, about 80 to 90. Once the tentacles have snared theprey, it is bitten with strong, beaklike paired jaws andpulled into the mouth by the tonguelike action of theradula.

Cephalopods have highly developed nervous systems,and their brains are unique among mollusks. Their eyes arevery elaborate, and have a structure much like that of verte-brate eyes, although they evolved separately (see chapter55). Many cephalopods exhibit complex patterns of behav-ior and a high level of intelligence; octopuses can be easilytrained to distinguish among classes of objects. Most mem-bers of this class have closed circulatory systems and are theonly mollusks that do.

Although they evolved from shelled ancestors, livingcephalopods, except for the few species of nautilus, lack anexternal shell. Like other mollusks, cephalopods take waterinto the mantle cavity and expel it through a siphon.Cephalopods have modified this system into a means of jetpropulsion. When threatened, they eject water violentlyand shoot themselves through the water.

Most octopus and squid are capable of changing color tosuit their background or display messages to one another.They accomplish this feat through the use of their chro-matophores, pouches of pigments embedded in the epithe-lium.

Gastropods typically live in a hard shell. Bivalves havehinged shells but do not have a distinct head area.Cephalopods possess well-developed brains and are themost intelligent invertebrates.


FIGURE 45.9A bivalve. The file shell, Lima scabra, opened, showing tentacles.

FIGURE 45.10A cephalopod. Squids are active predators, competing effectivelywith fish for prey.

Segmented AnimalsA key transition in the animal bodyplan was segmentation, the building of abody from a series of similar segments.The first segmented animals to evolvewere most likely annelid worms, phy-lum Annelida (figure 45.11). One ad-vantage of having a body built from re-peated units (segments) is that thedevelopment and function of theseunits can be more precisely controlled,at the level of individual segments orgroups of segments. For example, dif-ferent segments may possess differentcombinations of organs or perform dif-ferent functions relating to reproduc-tion, feeding, locomotion, respiration,or excretion.

Annelids

Two-thirds of all annelids live in the sea (about 8000species), and most of the rest, some 3100 species, areearthworms. Annelids are characterized by three principalfeatures:

1. Repeated segments. The body of an annelid wormis composed of a series of ring-like segments runningthe length of the body, looking like a stack of donutsor roll of coins (figure 45.12). Internally, the seg-ments are divided from one another by partitionscalled septa, just as bulkheads separate the segmentsof a submarine. In each of the cylindrical segments,the excretory and locomotor organs are repeated.The fluid within the coelom of each segment createsa hydrostatic (liquid-supported) skeleton that givesthe segment rigidity, like an inflated balloon. Muscleswithin each segment push against the fluid in thecoelom. Because each segment is separate, each canexpand or contract independently. This lets the wormmove in complex ways.

2. Specialized segments. The anterior (front) seg-ments of annelids have become modified to containspecialized sensory organs. Some are sensitive tolight, and elaborate eyes with lenses and retinashave evolved in some annelids. A well-developedcerebral ganglion, or brain, is contained in one an-terior segment.

3. Connections. Although partitions separate the seg-ments, materials and information do pass betweensegments. Annelids have a closed circulatory systemthat carries blood from one segment to another. A

ventral nerve cord connects the nerve centers or gan-glia in each segment with one another and the brain.These neural connections are critical features thatallow the worm to function and behave as a unifiedand coordinated organism.

Body Plan of the Annelids

The basic annelid body plan is a tube within a tube, withthe internal digestive tract—a tube running from mouth toanus—suspended within the coelom. The tube that makesup the digestive tract has several portions—the pharynx,esophagus, crop, gizzard, and intestine—that are special-ized for different functions.

Annelids make use of their hydrostatic skeleton for lo-comotion. To move, annelids contract circular musclesrunning around each segment. Doing so squeezes thesegment, causing the coelomic fluid to squirt outwards,like a tube of toothpaste. Because the fluid is trapped inthe segment by the septa, instead of escaping like tooth-paste, the fluid causes the segment to elongate and getmuch thinner. By then contracting longitudinal musclesthat run along the length of the worm, the segment is re-turned to its original shape. In most annelid groups, eachsegment typically possesses setae, bristles of chitin thathelp anchor the worms during locomotion. By extendingthe setae in some segments so that they anchor in thesubstrate and retracting them in other segments, theworm can squirt its body, section by section, in eitherdirection.



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FIGURE 45.11A polychaete annelid. Nereis virens is awide-ranging, predatory, marine polychaeteworm equipped with feathery parapodia formovement and respiration, as well as jawsfor hunting. You may have purchased Nereisas fishing bait!

Unlike the arthropods and most mollusks, most annelidshave a closed circulatory system. Annelids exchange oxygenand carbon dioxide with the environment through their bodysurfaces; most lack gills or lungs. However, much of theiroxygen supply reaches the different parts of their bodiesthrough their blood vessels. Some of these vessels at the ante-rior end of the worm body are enlarged and heavily muscular,serving as hearts that pump the blood. Earthworms have fivepulsating blood vessels on each side that serve as hearts, help-ing to pump blood from the main dorsal vessel, which is theirmajor pumping structure, to the main ventral vessel.

The excretory system of annelids consists of ciliated,funnel-shaped nephridia generally similar to those of mol-lusks. These nephridia—each segment has a pair—collectwaste products and transport them out of the body throughthe coelom by way of specialized excretory tubes.

Annelids are a diverse group of coelomate animalscharacterized by serial segmentation. Each segment inthe annelid body has its own circulatory, excretory,neural elements, and setae.


PHYLUM ANNELIDA: Segmentation

Segments

Setae

Clitellum

Mouth

Brain

Pharynx

Hearts

Esophagus Dorsalbloodvessel

Intestine

LongitudinalmuscleSepta

Male gonads

Femalegonads

Nervecord Ventral

bloodvessel NephridiumCircular

muscle

Each segment containsa set of excretoryorgans and a nerve center.

Segments are connected by the circulatoryand nervous systems. A series of heartsat the anterior (front) end pump the blood.A well-developed brain located in an anteriorsegment coordinates the activities of allsegments.

Each segment has a coelom. Muscles squeezethe fluid of the coelom, making each segmentrigid, like an inflated balloon. Because each segment can contract independently, a wormcan crawl by lengthening some segmentswhile shortening others.

Earthworms crawl byanchoring bristles calledsetae to the ground andpulling against them. Polychaete annelids have a flattened body and swimor crawl by flexing it.

FIGURE 45.12The evolution of segmentation. Marine polychaetes and earthworms (phylum Annelida) were most likely the first organisms to evolve abody plan based on partly repeated body segments. Segments are separated internally from each other by septa.

Classes of AnnelidsThe roughly 12,000 described species of annelids occur inmany different habitats. They range in length from as lit-tle as 0.5 millimeter to the more than 3-meter length ofsome polychaetes and giant Australian earthworms. Thereare three classes of annelids: (1) Polychaeta, which arefree-living, almost entirely marine bristleworms, comprisingsome 8000 species; (2) Oligochaeta, terrestrial earthwormsand related marine and freshwater worms, with some 3100species; and (3) Hirudinea, leeches, mainly freshwaterpredators or bloodsuckers, with about 500 species. The an-nelids are believed to have evolved in the sea, with poly-chaetes being the most primitive class. Oligochaetes seem tohave evolved from polychaetes, perhaps by way of brackishwater to estuaries and then to streams. Leeches share witholigochaetes an organ called a clitellum, which secretes acocoon specialized to receive the eggs. It is generally agreedthat leeches evolved from oligochaetes, specializing in theirbloodsucking lifestyle as external parasites.

Class Polychaeta: The Polychaetes

Polychaetes (class Polychaeta) include clamworms, plumeworms, scaleworms, lugworms, twin-fan worms, sea mice,peacock worms, and many others. These worms are oftensurprisingly beautiful, with unusual forms and sometimesiridescent colors (figure 45.13; see also figure 45.1). Poly-chaetes are often a crucial part of marine food chains, asthey are extremely abundant in certain habitats.

Some polychaetes live in tubes or permanent burrows ofhardened mud, sand, mucuslike secretions, or calcium car-bonate. These sedentary polychaetes are primarily filterfeeders, projecting a set of feathery tentacles from the tubesin which they live that sweep the water for food. Otherpolychaetes are active swimmers, crawlers, or burrowers.Many are active predators.

Polychaetes have a well-developed head with specializedsense organs; they differ from other annelids in this re-spect. Their bodies are often highly organized into distinctregions formed by groups of segments related in functionand structure. Their sense organs include eyes, which rangefrom simple eyespots to quite large and conspicuous stalkedeyes.

Another distinctive characteristic of polychaetes is thepaired, fleshy, paddlelike flaps, called parapodia, on mostof their segments. These parapodia, which bear bristlelikesetae, are used in swimming, burrowing, or crawling. Theyalso play an important role in gas exchange because theygreatly increase the surface area of the body. Some poly-chaetes that live in burrows or tubes may have parapodiafeaturing hooks to help anchor the worm. Slow crawling iscarried out by means of the parapodia. Rapid crawling andswimming is by undulating the body. In addition, the poly-chaete epidermis often includes ciliated cells which aid inrespiration and food procurement.

The sexes of polychaetes are usually separate, and fertil-ization is often external, occurring in the water and awayfrom both parents. Unlike other annelids, polychaetes usu-ally lack permanent gonads, the sex organs that producegametes. They produce their gametes directly from germcells in the lining of the coelom or in their septa. Fertiliza-tion results in the production of ciliated, mobile trochophorelarvae similar to the larvae of mollusks. The trochophoresdevelop for long periods in the plankton before beginningto add segments and thus changing to a juvenile form thatmore closely resembles the adult form.

Class Oligochaeta: The Earthworms

The body of an earthworm (class Oligochaeta) consists of100 to 175 similar segments, with a mouth on the first andan anus on the last. Earthworms seem to eat their waythrough the soil because they suck in organic and othermaterial by expanding their strong pharynx. Everythingthat they ingest passes through their long, straight digestivetracts. One region of this tract, the gizzard, grinds up theorganic material with the help of soil particles.

The material that passes through an earthworm is de-posited outside of its burrow in the form of castings that


FIGURE 45.13A polychaete. The shiny bristleworm, Oenone fulgida.

consist of irregular mounds at the opening of a burrow. Inthis way, earthworms aerate and enrich the soil. A wormcan eat its own weight in soil every day.

In view of the underground lifestyle that earthwormshave evolved, it is not surprising that they have no eyes.However, earthworms do have numerous light-, chemo-,and touch-sensitive cells, mostly concentrated in segmentsnear each end of the body—those regions most likely to en-counter light or other stimuli. Earthworms have fewersetae than polychaetes and no parapodia or head region.

Earthworms are hermaphroditic, another way in whichthey differ from most polychaetes. When they mate (figure45.14), their anterior ends point in opposite directions, andtheir ventral surfaces touch. The clitellum is a thickenedband on an earthworm’s body; the mucus it secretes holdsthe worms together during copulation. Sperm cells are re-leased from pores in specialized segments of one partnerinto the sperm receptacles of the other, the process goingin both directions simultaneously.

Two or three days after the worms separate, the clitel-lum of each worm secretes a mucous cocoon, surroundedby a protective layer of chitin. As this sheath passes overthe female pores of the body—a process that takes place asthe worm moves—it receives eggs. As it subsequentlypasses along the body, it incorporates the sperm that weredeposited during copulation. Fertilization of the eggs takesplace within the cocoon. When the cocoon finally passesover the end of the worm, its ends pinch together. Withinthe cocoon, the fertilized eggs develop directly into youngworms similar to adults.

Class Hirudinea: The Leeches

Leeches (class Hirudinea) occur mostly in fresh water, al-though a few are marine and some tropical leeches occupyterrestrial habitats. Most leeches are 2 to 6 centimeterslong, but one tropical species reaches up to 30 centimeters.Leeches are usually flattened dorsoventrally, like flat-worms. They are hermaphroditic, and develop a clitellumduring the breeding season; cross-fertilization is obligatoryas they are unable to self-fertilize.

A leech’s coelom is reduced and continuous throughoutthe body, not divided into individual segments as in thepolychaetes and oligochaetes. Leeches have evolved suckersat one or both ends of the body. Those that have suckers atboth ends move by attaching first one and then the otherend to the substrate, looping along. Many species are alsocapable of swimming. Except for one species, leeches haveno setae.

Some leeches have evolved the ability to suck bloodfrom animals. Many freshwater leeches live as external par-asites. They remain on their hosts for long periods andsuck their blood from time to time.

The best-known leech is the medicinal leech, Hirudomedicinalis (figure 45.15). Individuals of Hirudo are 10 to 12centimeters long and have bladelike, chitinous jaws that

rasp through the skin of the victim. The leech secretes ananticoagulant into the wound to prevent the blood fromclotting as it flows out, and its powerful sucking musclespump the blood out quickly once the hole has been opened.Leeches were used in medicine for hundreds of years tosuck blood out of patients whose diseases were mistakenlybelieved to be caused by an excess of blood. Today, Euro-pean pharmaceutical companies still raise and sell leeches,but they are used to remove excess blood after certain surg-eries. Following the surgery, blood may accumulate be-cause veins may function improperly and fail to circulatethe blood. The accumulating blood “turns off” the arterialsupply of fresh blood, and the tissue often dies. Whenleeches remove the excess blood, new capillaries form inabout a week, and the tissues remain healthy.

Segmented annelids evolved in the sea. Earthworms aretheir descendents, as are parasitic leeches.


FIGURE 45.14Earthworms mating. The anterior ends are pointing in oppositedirections.

FIGURE 45.15Hirudo medicinalis, the medicinal leech, is seen here feeding on ahuman arm. Leeches uses chitinous, bladelike jaws to make anincision to access blood and secrete an anticoagulant to keep theblood from clotting. Both the anticoagulant and the leech itselfhave made important contributions to modern medicine.

LophophoratesThree phyla of marine animals—Phoronida, Ectoprocta,and Brachiopoda—are characterized by a lophophore, acircular or U-shaped ridge around the mouth bearing oneor two rows of ciliated, hollow tentacles. Because of thisunusual feature, they are thought to be related to one an-other. The lophophore presumably arose in a common an-cestor. The coelomic cavity of lophophorates extends intothe lophophore and its tentacles. The lophophore functionsas a surface for gas exchange and as a food-collectionorgan. Lophophorates use the cilia of their lophophore tocapture the organic detritus and plankton on which theyfeed. Lophophorates are attached to their substrate ormove slowly.

Lophophorates share some features with mollusks, an-nelids and arthropods (all protostomes) and share otherswith deuterostomes. Cleavage in lophophorates is mostlyradial, as in deuterostomes. The formation of the coelomvaries; some lophophorates resemble protostomes in thisrespect, others deuterostomes. In the Phoronida, themouth forms from the blastopore, while in the other twophyla, it forms from the end of the embryo opposite theblastopore. Molecular evidence shows that the ribosomesof all lophophorates are decidedly protostome-like, lendingstrength to placing them within the protostome phyla. De-spite the differences among the three phyla, the uniquestructure of the lophophore seems to indicate that themembers share a common ancestor. Their relationshipscontinue to present a fascinating puzzle.

Phylum Phoronida: The Phoronids

Phoronids (phylum Phoronida) superficially resemblecommon polychaete tube worms seen on dock pilings buthave many important differences. Each phoronid secretesa chitinous tube and lives out its life within it (figure45.16). They also extend tentacles to feed and quicklywithdraw them when disturbed, but the resemblance tothe tube worm ends there. Instead of a straight tube-within-a-tube body plan, phoronids have a U-shaped gut.Only about 10 phoronid species are known, ranging inlength from a few millimeters to 30 centimeters. Somespecies lie buried in sand, others are attached to rocks ei-ther singly or in groups. Phoronids develop as proto-stomes, with radial cleavage and the anus developing sec-ondarily.

Phylum Ectoprocta: The Bryozoans

Ectoprocts (phylum Ectoprocta) look like tiny, short ver-sions of phoronids (figure 45.17). They are small—usually

less than 0.5 millimeter long—and live in colonies that looklike patches of moss on the surfaces of rocks, seaweed, orother submerged objects (in fact, their common name bry-ozoans translates from Greek as “moss-animals”). Thename Ectoprocta refers to the location of the anus (proct),which is external to the lophophore. The 4000 species in-clude both marine and freshwater forms—the only nonma-rine lophophorates. Individual ectoprocts secrete a tinychitinous chamber called a zoecium that attaches to rocksand other members of the colony. Individuals communicatechemically through pores between chambers. Ectoproctsdevelop as deuterostomes, with the mouth developing sec-ondarily; cleavage is radial.



LophophoreTentacles oflophophore

Anus

Mouth

Nephridium

Body wall

Intestine

Testis

Ovary

FIGURE 45.16Phoronids (phylum Phoronida). A phoronid, such as Phoronis,lives in a chitinous tube that the animal secretes to form the outerwall of its body. The lophophore consists of two parallel,horseshoe-shaped ridges of tentacles and can be withdrawn intothe tube when the animal is disturbed.

Phylum Brachiopoda: TheBrachiopods

Brachiopods, or lamp shells, superfi-cially resemble clams, with two calci-fied shells (figure 45.18). Manyspecies attach to rocks or sand by astalk that protrudes through an open-ing in one shell. The lophophore lieswithin the shell and functions whenthe brachiopod’s shells are openedslightly. Although a little more than300 species of brachiopods (phylumBrachiopoda) exist today, more than30,000 species of this phylum areknown as fossils. Because brachiopodswere common in the earth’s oceansfor millions of years and because theirshells fossilize readily, they are oftenused as index fossils to define a partic-ular time period or sediment type.Brachiopods develop as deutero-stomes and show radial cleavage.

The three phyla of lophophoratesprobably share a commonancestor, and they show a mixtureof protostome and deuterostomecharacteristics.


Anus

Intestine

Mouth

Stomach

Lophophore

Retractedlophophore

Retractormuscle

Zoecium

(a) (b)

FIGURE 45.17Ectoprocts (phylum Ectoprocta). (a) A small portion of a colony of the freshwater ectoproct, Plumatella (phylum Ectoprocta),which grows on the underside of rocks. The individual at the left has a fully extended lophophore, the structure characteristic ofthe three lophophorate phyla. The tiny individuals of Plumatella disappear into their shells when disturbed. (b) Plumatella repens, afreshwater bryozoan.

Pedicel

Digestivegland

Stomach

Nephridium

Intestine

Muscle

CoelomMouth Spiral portion of lophophore

Lateral arm of lophophoreMantle

Ventral (pedicel)valve

Dorsal (brachial)valve

Gonad

FIGURE 45.18Brachiopods (phylumBrachiopoda). (a) Thelophophore lies within twocalcified shells, or valves.(b) The brachiopod,Terebratolina septentrionalsi, isshown here slightly opened sothat the lophophore is visible.

(a)

(b)


Chapter 45 Summary Questions Media Resources


• Mollusks contain a true body cavity, or coelom,within the embryonic mesoderm and were among thefirst coelomate animals.

• The mollusks constitute the second largest phylum ofanimals in terms of named species. Their body planconsists of distinct parts: a head, a visceral mass, and afoot.

• Of the seven classes of mollusks, the gastropods(snails and slugs), bivalves (clams and scallops), andcephalopods (octopuses, squids, and nautilus), arebest known.

• Gastropods typically live in a hard shell. Duringdevelopment, one side of the embryo grows morerapidly than the other, producing a characteristictwisting of the visceral mass.

• Members of the class Bivalvia have two shells hingedtogether dorsally and a wedge-shaped foot. They lackdistinct heads and radulas. Most bivalves are filter-feeders.

• Octopuses and other cephalopods are efficient andoften large predators. They possess well-developedbrains and are the most intelligent invertebrates.

1. What is the basic body plan ofa mollusk? Where is the mantlelocated? Why is it important inthe mollusks? What occurs inthe mantle cavity of aquaticmollusks?2. What is a radula? Do allclasses of mollusks possess thisstructure? How is it used indifferent types of mollusks?3. How does the molluskexcretory structure work? Whyis it better than the flame cells ofacoelomates?4. What is a trochophore? Whatis a veliger?5. Do bivalves generally disperseas larvae or adults? Explain.

• Segmentation is a characteristic seen only incoelomate animals at the annelid evolutionary leveland above. Segmentation, or the repetition of bodyregions, greatly facilitates the development ofspecialized regions of the body.

• Annelids are worms with bodies composed ofnumerous similar segments, each with its owncirculatory, excretory, and neural elements, and arrayof setae. There are three classes of annelids, thelargely marine Polychaeta, the largely terrestrialOligochaeta, and the largely freshwater Hirudinea.

6. What evolutionary advantagesdoes segmentation confer uponan organism?7. What are annelid setae? Whatfunction do they serve? Whatare parapodia? What class ofannelids possess them?8. How do earthworms obtaintheir nutrients? What sensorystructures do earthwormspossess? How do these animalsreproduce?


• The lophophorates consist of three phyla of marineanimals—Phoronida, Ectoprocta, and Brachiopoda—characterized by a circular or U-shaped ridge, thelophophore, around the mouth.

• Some lophophorates have characteristics likeprotostomes, others like deuterostomes. All arecharacterized by a lophophore and are thought toshare a common ancestor.

9. What prominent featurecharacterizes the lophophorateanimals? What are the functionsof this feature?


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