12Annelida: TheMetameric Body FormThe Samoan palolo worm (Eunice viridis) is one of approximately 15,000 spe cies in the phylum Annelida. The palolo's 1·eproductive habits are unusual, buteffective. As you will see in this chapter, "unusual" is not the exception for thistaxonomically challenging phylum.Chapter Outline12.1 Evolutionary PerspectiveRelationships to Other AnimalsMetamerism and Tagmatization12.2 Annelid Structure and FunctionExternal Structure and LocomotionFeeding and the Digestive SystemGas Exchange and CirculationNervous and Sensory FunctionsExcretionRegeneration, Reproduction,and Development12.3 Clade (Class) ErrantiaNereis (Neanthes, Alitta)GlyceraFireworms12.4 Clade (Class) SedentariaTubewormsSiboglinidaeEchiuraClade Clitel/ata12.5 Basal Annelid GroupsChaetopteridaeSipuncula12.6 Further Phylogenetic Considerations12. 1EVOLUTIONARY PERSPECTIVELEARNING OUTCOMES1. Describe the relationships of members of the Annelida to other animal phyla.2. Explain the benefits of metamerism for an annelidAt the time of the November full moon on islands near Samoa in the SouthPacific, people rush about preparing for one of their biggest yearly feasts. Injust one week, the sea will yield a harvest that can be scooped up in nets andbuckets (figure 12.1). Worms by the millions transform the ocean into what onewriter called "vermicelli soup!" Celebrants gorge themselves on worms that havebeen cooked or wrapped in breadfruit leaves. The Samoan palolo worm (Euniceviridis, alternatively Palo/a viridis) spends its entire adult life in coral burrowsat the sea bottom. Each November, one week after the full moon, this wormemerges from its burrow, and specialized body segments devoted to sexualreproduction break free and float to the surface, while the rest of the wormis safe on the ocean floor. The surface water is discolored as gonads releasetheir countless eggs and sperm. The natives' feast is short lived, however; thesereproductive swarms last only two days and do not recur for another year.The Samoan palolo worm is a member of the phylum Annelida (ah-nel'i-dah)(L. annellus, ring). Other members of this phylum include countless marineworms, the soil-building earthworms, and predatory leeches (table 12.1).Characteristics of the phylum Annelida include:1. Body metameric, bilaterally symmetrical, and worm-like2. Spiral cleavage, trochophore larvae (when larvae are present), andschizocoelous coelom formation3. Paired, epidermal setae (chaetae)4. Closed circulatory system5. Dorsal suprapharyngeal ganglia and ventral nerve cord(s) with ganglia6. Metanephridia (usually) or protonephridiaRelationships to Other AnimalsIt has been clear for many years that Annelida is a monophyletic assemblage ofmarine, freshwater, and terrestrial worms. They are lophotrochozoans and, thus,share common ancestry with Mollusca, Brachiopoda, Bryozoa, Nemertea, and others
Annelida: The Metameric Body FormFIGURE 12.1(a)221(b)Palolo Feasts. (a) Women from the Nggela Group (Florida Islands) of the Solomon Islands use light from a torch to attract reproductivesegments of Eunice viridis (family Eunicidae) during collecting. Photoreceptors on the reproductive segments elicit the response of theworms to light. (b) A swarm of worms as viewed through the lens of a diver's camera. The photograph on page 220 shows palolo worms(referred to by islanders as "odu") ready for feasting. (Pbotograpbs courte.,y ofDr. Si111011 Foale, Principle Resectrcb Fe/10111, ARC Ce11tre of Exce/le11cefor Coral Req(Studies.James Cook University, Quee11sla11d, Austmlio.)ProtistsBasalPhylaFIGURE 12.2Evolutionary Relationships of Annelids to Other Animals. This figure shows an interpretation of the relationship of the Annelida toother members of the animal kingdom. The relationships depicted here are based on evidence from developmental and molecular biology.Annelids are placed within the Lophotrochozoa along with the Mollusca, Plathyhelminthes, Rotifera, and others (see pages xvi-xvii). Thephylum includes approximately 15,000 species of segmented worms. Most of these are marine and traditionally classified into the classPolychaeta. As discussed in the text, this placement is being reevaluated based on evidence from molecular biology. The Christmas-treeworm (Spirohrcmcbus gigan/e11s) (family Serpulidae) shown here is a memher of the clade Senclentaria The spiral fans of this tube-dwellingannelid are derived from prostomial palps that surround the mouth and are specialized for feeding and gas exchange.
222CHAPTER TWELVETABLE 12.1CLASSIFICATION OF THE PHYLUM ANNELIDAPhylum Annelida (ah-nel'i-dah)The phylum of triploblastic, coelomate animals whose membersare metameric (segmented), elongate, and cylindrical oroval in cross section. Annelids have a complete digestivetract; paired, epidermal setae; and a ventral nerve cord.Approximately 15,000 species of annelids have beendescribed.Clade (Class) Errantia (er-ran'tiah)Marine annelids; parapodia with prominent lobes supportedby internalized chaetae and ventral cirri; palps welldeveloped. Nereis, Eunice, Glycera.Clade (Class) Sedentaria (sed-en-ter'iah)Marine, freshwater, and terrestrial annelids; parapodia withreduced lobes or parapodia lacking; setae associatedwith the stiff body wall to facilitate anchoring in tubesand burrows; palps reduced. Includes the clade Clitellata,echiruans, and siboglinids. Arenicola, Riftia, Lumbricus,HirudoOther Annelid TaxaSipuncula (si-pun'ku-lah)Marine; unsegmented body; retractable anterior trunkcalled an introvelt. Formerly phylum Sipuncula. Inclusionof this group within the Annelida remains somewhatcontroversial. Themiste.Chaetopteridae (ke-top-ter'ida)Marine; three-palt body; lives in a U-shaped tube,appendages for feeding and creating water currents.Chaetoptei·us.(figure 12. 2 and see chapters 10 and 11). Our understandingof phylogeny within the phylum, however, has a history ofcontentious debate. The application of modern phylogeneticanalysis is helping unravel the annelid taxonomic-tangle.Anyone who has followed the debates involved with elu cidating annelid phylogeny will have a better appreciationof the dynamics within the field of animal taxonomy. Thesedebates underscore the importance of taxonomy in helpingzoologists understand the evolution of bilateral morphology(see the discussion of metamerism and tagmatization in thenext section).Previous editions of this book described the Annelidaas being composed of two classes: Polychaeta (marineworms) and Clitellata (leeches, earthworms, and others).While "Polychaeta" was rightly described as being paraphy letic, recent molecular phylogenetic work has shown thatthe entire annelid assemblage is encompassed by what wasbeing called "Polychaeta." As will be discussed in moredetail at the end of this chapter, "Polychaeta" has beenrevealed to be synonymous with Annelida, and the formershould be discarded as a class name. Two older terms usedto differentiate groups within the polychaetes have been res urrected to represent two major annelid clades: Errantia andSedentaria. Sedentaria now includes some "polychaetes,"leeches, earthworms, and even worms that were formerlyconsidered separate phyla (Echiura and Pogonophora). Cli tellata is still a valid clade within the Sendentaria composedof earthworms, leeches, and a few smaller taxa. The leechesform a monophyletic clade (Hirudinea) within the Clitellata.The earthworms and their relatives, however, are not mono phyletic. Thus the name "Oligochaeta," which was formerlyconsidered a subclass name within the Clitellata, shouldbe abandoned as a taxonomic designation. Two othergroups lie outside of Sedentaria and Errantia. One of these,Chaetopteridae, was formerly considered to be a familywithin the "Polychaeta." The other, Sipuncula, was anotherphylum outside of Annelida. The hierarchical taxonomic categories associated with these clades have not been estab lished. We treat Errantia and Sedentaria as clades with aparenthetical "class" designation, which seems a logical out come of ongoing and future research.Chapter 12 is now organized to reflect this new phylo genetic work. In the next section, we describe metamerismand tagmatization. This discussion is followed by coverageof annelid structure and function. The term "polychaete" isused in a nontaxonomic sense to refer to a host of marineannelids (both errantians and sedentarians) when adap tations related to their common marine habitat result insimilar structures and functions. After the basics of anne lid structure and function are discussed, the two majorannelid clades and the two outlying groups are described.Since members of the clade Clitellata are different in manyrespects from other sedentarians, unique aspects of theirstructure and function are described in the section on Sed entaria. As always, this chapter ends with "Further Phyloge netic Considerations."Metamerism and TagmatizationEarthworm bodies are organized into a series of ringlike seg ments. What is not externally obvious, however, is that thebody is divided internally as well. Segmental arrangement ofbody parts in an animal is called metamerism (Gr. meta,after mere, part).Metamerism profoundly influences virtually eve1yaspect of annelid structure and function, such as the anatomi cal arrangement of organs that are coincidentally associatedwith metamerism. For example, the compartmentalizationof the body has resulted in each segment having its ownexcreto1y, nervous, and circulato1y structures. In most mod ern annelids two related functions are probably the primaryadaptive features of metamerism: flexible support and effi cient locomotion. These functions depend on the metamericarrangement of the coelom and can be understood by exam ining the development of the coelom and the arrangement ofbody-wall muscles.
Annelida: The Metameric Body FormDuring embryonic development, the body cavity ofannelids arises by a segmental splitting of a solid mass ofmesoderm that occupies the region between ectoderm andendoderm on either side of the embryonic gut tract. Enlarge ment of each cavity forms a double-membraned septum onthe anterior and posterior margins of each coelomic spaceand dorsal and ventral mesenteries associated with the diges tive tract (figure 12.3).Muscles also develop from the mesodermal layers associ ated with each segment. A layer of circular muscles lies belowthe epidermis, and a layer of longitudinal muscles, just belowthe circular muscles, runs between the septa that separateEndodermMesodermEctoderm223each segment. In addition, some marine annelids have obliquemuscles, and the leeches have dorsoventral muscles.One advantage of the segmental arrangement of coe lomic spaces and muscles is the creation of hydrostatic com partments, which allow a variety of advantageous locomotorand supportive functions not possible in nonmetameric ani mals that use a hydrostatic skeleton. Each segment can becontrolled independently of distant segments, and musclescan act as antagonistic pairs within a segment. The constantvolume of coelomic fluid provides a hydrostatic skeletonagainst which muscles operate. Resultant localized changesin the shape of groups of segments provide the basis forswimming, crawling, and burrowing.A second advantage of metamerism is that it lessens theimpact of injury. If one or a few segments are injured, adjacentsegments, set off from injured segments by septa, may be able tomaintain nearly normal functions, which increases the likelihoodthat the worm, or at least a part of it, will survive the trauma.A third advantage of metamerism is that it permits themodification of certain regions of the body for specializedfunctions, such as feeding, locomotion, and reproduction.The specialization of body regions in a metameric animal iscalled tagmatization (Gr. tagma, arrangement). The special ization of posterior segments of the palolo worm for repro ductive functions (see figure 12.1) is an example of annelidtagmatization. Metamerism is not unique to the Annelida.It is also present in the Arthropoda (insects, arachnids, andtheir relatives) and Chordata (vertebrates, including humans).Interestingly metamerism is absent in the annelid taxa Echi ura and Sipuncula. It was probably lost in these lineages.This evolutionary convergence of the metameric body formin these three very successful phyla means that one or moreof the previously described advantages have been a majorinfluence in the evolution of animal body forms.CoalomSECTION REVIEW12.1Members of the phylum Annelida are the segmented worms.They are lophotrocozoans and thus share ancestry with themolluscs, nemerteans, rotifers, and others. Annelid bodies aremetameric. Metamerism results in separate hydrostatic compart ments, reduces the impact of injury, and permits tagmatization.Ventral mesenterySeptumFIGURE 12.3Development of Metameric, Coelomic Spaces in Annelids.(a) A solid mesodermal mass separates ectoderm and endodermin early embryological stages. (b) Two cavities in each segmentform from the mesoderm splitting on each side of the endoderm(schizocoelous coelom formation). (c) These cavities spread in alldirections. Enlargement of the coelomic sacs leaves a thin layerof mesoderm applied against the outer body wall (the parietalperitoneum) and the gut tract (the visceral peritoneum), and dorsaland ventral mesenteries form. Anterior and posterior expansionof the coelom in adjacent segments forms the double-membranedseptum that separates annelid metameres.How bas evidence from molecular biology forced thereevaluation of classification of the Annelida?12.2ANNELID STRCCTUREAND FUNCTIO LEARNING OUTCOMES1. Explain how metamerism influences the biology ofannelid worms.2. Compare the closed circulatory system of an annelidworm to the open circulatory system of a bivalve mollusc.
224CHAPTER TWELVEPolychaetes are mostly marine, and are usually between 5 and10 cm long. Polychaetes have adapted to a variety of habitats.Errantian polychaetes live on the ocean floor, under rocksand shells, and within the crevices of coral reefs. Many mem bers of both major annelid clades are burrowers and movethrough their substrate by peristaltic contractions of the bodywaIT:- A bucket of imertidal :;;an I normallyyielcfs vast numoersand an amazing variety of these burrowing annelids. Manysedentarians construct tubes of cemented sand grains orsecreted organic materials. Mucus-lined tubes serve as pro tective retreats and feeding stations. Other annelids, likeoligochaetes and leeches, have adapted to freshwater and ter restrial environments.Sedentarians and errantians share many features ofstructure and function because they share a common ances try. On the other hand, adaptations to diverse environmentswithin these groups mean that there are many variations onthe annelid theme. These common features, and many varia tions, will be described as we progress through this section.External Structure and LocomotionIn addition to metamerism, the most distinctive feature ofmany annelids is the presence of lateral extensions calledparapodfa (Gr. para, oeside podion, little foot) (figure 12.4).In the Errantia, chitinous rods support the parapodia, andnumerous setae project from the parapodia. Parapodia arereduced or absent (clade Clitellata) in the Sedentaria. Setaeare present in most sedentarians but they are in closer prox imity to the body wall. (Setae are absent in most leeches.)Setae (L. saeta, bristle) (also called chaetae) are bristlessecreted from invaginations of the distal ends of parapodia.They aid locomotion by digging into the substrate (Errantia)and also hold a worm in its burrow or tube (Sedentaria).FIGURE 12.4Clade Errantia. External structure of Nereis virens. Note thenumerous parapodia.The prostomium (Gr. pro, before stoma, mouth) ofa polychaete is a lobe that projects dorsally and anteriorly tothe mouth and contains numerous sensory structures, includingeyes, antennae, palps, and ciliated pits or grooves, called nuchalorgans. The palps of some sedentarians are highly modifiedinto filtering fan-like structures (see figures 12.2 and 12.12).The fiisCfoaysegment,- theperistomium (Gr. peii: arounci)-,surrounds the mouth and bears sensory tentacles or cirri.The epidermis of annelids consists of a single layer ofcolumnar cells that secrete a protective, nonliving cuticle.Some annelids have epidermal glands that secrete lumines cent compounds.Various species of errantian annelids are capable ofwalking, fast crawling, or swimming. To enable them to doso, the longitudinal muscles on one side of the body actantagonistically to the longitudinal muscles on the otherside of the body so that undulatory waves move along thelength of the body from the posterior end toward the head.The propulsive force is the result of parapodia and setaeacting against the substrate or water. Parapodia on oppositesides of the body are out of phase with one another. Whenlongitudinal muscles on one side of a segment contract, theparapodial muscles on that side also contract, stiffening theparapodium and protruding the setae for the power stroke(figure 12.5a). As a polychaete changes from a slow crawl toswimming, the period and amplitude of undulatory wavesincrease (figure 12.5b).Burrowing sedentarians push their way through sandand mud by contractions of the body wall or by eating theirway through the substrate. In the latter, the annelids digestorganic matter in the substrate and eliminate absorbed andundigestible materials via the anus.Feeding and the Digestive SystemThe digestive tract of most annelids is a straight tube thatmesenteries and septa suspend in the body cavity. The ante rior region of the digestive tract is modified into a proboscisthat special protractor muscles and coelomic pressure caneve1t through the mouth. Retractor muscles bring the probos cis back into the peristomium. In some, when the proboscisis everted, paired jaws are opened and may be used for seiz ing prey. Predatory species may not leave their burrow orcoral crevice. When prey approaches a burrow entrance, theworm quickly extends its anterior portion, everts the probos cis, captures prey with its jaws, and pulls the prey back intothe burrow. Some annelids have poison glands at the base ofthe jaw. Other annelids are herbivores and scavengers anduse jaws for tearing food. Deposit-feeding polychaetes (e.g.,Arenicola, the sedentarian lugworm) extract organic matterfrom the marine sediments they ingest. The digestive tractconsists of a pharynx that, when everted, forms the proboscis;a storage sac, called a crop; a grinding gizzard; and a long,straight intestine (seefigure 12.12). Organic matter is digestedextracellularly, and the inorganic particles are passed throughthe intestine and released as "castings."
Annelida: The Metameric Body FormLongitudinal muscles relaxedand incompletely stretched225Longitudinal musclesfully stretchedParapodial muscles relax,resulting in withdrawal of --.arparapodia and setaeLongitudinal musclesfully contractedLongitudinal musclesfully contracted-- - Points of contact withsubstrate or mediumParapodial musclescontract, protrudingparapodia and setaeLongitudinal musclesfully stretched(a)6-8 segrnentsaverage"wavelength"(b)14 segmen1s"wavelength"Slow walking1 40 segmerils"wavelength"Rapid crawlingSwimmingFIGURE 12.5Annelid Locomotion. (a) Dorsal view of a primitive errantian annelid, showing the antagonism of longitudinal muscles on opposite sidesof the body and the resultant protrusion and movement of parapodia. (b) Both the period and amplitude of locomotor waves increase as theannelid changes from a "slow walk" to a swimming mode. From: ''.A LIFE OF INVERTEBRATES" 1979 w. D. Russell-Hunter.Many sedentary and tube-dwelling polychaetes are fil ter feeders. They usually lack a proboscis but possess otherspecialized feeding structures. Some tube dwellers, called fan worms, possess radioles that form a spiral-shaped or funnel shaped fan (see figures 12.2 and 12.12). Cilia on the radiolescirculate water through the fan, trapping food particles.Trapped patticles are carried along a food groove at the axisof the radiole. During transpott, a so1ting mechanism rejectsthe largest particles and transports the finest particles to themouth. Another filter feeder, Chaetopterus, lives in a U-shapedtube and secretes a mucous bag that collects food patticles,which may be as small as 1 µm. The parapodia of segments 14through 16 are modified into fans that create filtration currents.When full, the entire mucous bag is ingested (see.figure 12.22).Elimination of digestive waste products can be a prob lem for tube-dwelling polychaetes. Those that live in tubesthat open at both ends simply have wastes carried away bywater circulating through the tube. Those that live in tubesthat open at one end must turn around in the tube to def ecate, or they may use cilia1y tracts along the body wall tocarry feces to the tube opening.Polychaetes that inhabit substrates rich in dissolvedorganic matter can absorb as much as 20 to 40% of theirenergy requirements across their body wall as sugars andother organic compounds. This method of feeding occurs inother animal phyla, too, but rarely accounts for more than 1%of their energy needs.Gas Exchange and CirculationThe respiratory gases of most annelids simply diffuse acrossthe body wall, and parapodia increase the surface area for theseexchanges. In many annelids, parapodial gills further increasethe surface area for gas exchange.Annelids have a closed circulatory system. Oxygen isusually carried in combination with molecules called respi ratory pigments, which are usually dissolved in the plasmarather than contained in blood cells. Blood may be color less, green, or red, depending on the presence and/or type ofrespirat01y pigment.Contractile elements of annelid circulatory systems con sist of a dorsal aorta that lies just above the digestive tractand propels blood from rear to front, and a ventral aorta thatlies ventral to the digestive tract and propels blood from frontto rear. Running between these two vessels are two or threesets of segmental vessels that receive blood from the ventralaorta and break into capillary beds in the gut and body wall.
226CHAPTER TWELVEGut wall mSensory -E---'11projectionsGiant fibersFIGURE 12.6Circulatory System of a Polychaete. Cross section through thebody and a parapodium. In the closed circulatory system shownhere, blood passes posterior to anterior in the dorsal vessel andanterior to posterior in the ventral vessel. The direction of bloodflow is indicated by black arrows. Capillary beds interconnectdorsal and ventral vessels.Small fibers(b) ,,FIGURE 12.7Capillaries coalesce again into segmental vessels that deliverblood to the dorsal aorta (figure 12.6; see figures 12.17and 12.18).Nervous and Sensory FunctionsNe1vous systems are similar in virtually all annelids. Theannelid nervous system includes a pair of suprapharyngealganglia, which connect to a pair of subpharyngeal ganglia bycircumpha1yngeal connectives that run dorsoventrally alongeither side of the pharynx. A double ventral nerve cord runsthe length of the worm along the ventral margin of each coe lomic sp,:ice, ,:ind a pairc d segmental g:mglion i.-, in earh seg ment. The double ventral ne1ve cord and paired segmentalganglia may fuse to va1ying extents in different taxonomicgroups. Lateral nerves emerge from each segmental ganglion,supplying the body-wall musculature and other structures ofthat segment (figure 12.7a).Segmental ganglia coordinate swimming and crawlingmovements in isolated segments. (Anyone who has used por tions of worms as live fish bait can confirm that the headend-with the pharyngeal ganglia-is not necessa1y for coor dinated movements.) Each segment acts separately from,but is closely coordinated with, neighboring segments. Thesubphatyngeal ganglia help mediate locomotor functionsrequiring coordination of distant segments. The suprapharyn geal ganglia probably control motor and sensory functionsinvolved with feeding, and sens01y functions associated withforward locomotion.In addition to small-diameter fibers that help coordi nate locomotion, the ventral nerve cord also contains giantfibers involved with escape reactions (figure 12.7b). Forexample, a harsh stimulus, such as a fishhook, at one endof a worm causes rapid withdrawal from the stimulus. Giantfibers are approximately 50 µm in diameter and conductAnnelid Nervous System. (a) Connectives link suprapha1yngealand subpha1yngeal ganglia. Segmental ganglia and lateral nervesoccur along the length of the worm. (b) Cross section of the ventralne1ve cord, showing giant fibers.nerve impulses at 30 m/s (as opposed to 0.5 m/s in thesmaller, 4-µm-diameter annelid fibers).Annelids have various senso1y structures. Two to fourpairs of eyes are on the surface of the prostomium. Theyva1y in complexity from dermal photoreceptor cells; to sim ple cups of receptor cells; to structures made up of a cornea,lens, and vitreous body. Most polychaetes react negatively toincreased light intensities. Fanworms, however, react nega tively to decreasing light intensities. If shadows cross them,fanworms retreat into their tubes. This response is believed tohelp protect fanworms from passing predators. Earthwormslack well-developed eyes, which is not surprising, given theirsubterranean lifestyle. They do possess a dermal light sensethat arises from photoreceptor cells scattered over dorsal andlateral surfaces of the body. Scattered photoreceptors mediatea negative phototaxis in strong light (evidenced by move ment away from the light source) and positive phototaxis inweak light (evidenced by movement toward the light source).Other sense organs mediate responses to chemicals, grav ity, and touch. Nuchal organs are pairs of ciliated sensory pits orslits in the head region. Neives from the suprapha1yngeal gan glia inne1vate nuchal organs, which are thought to be chemo receptors for food detection. Statocysts are in the head regionof polychaetes, and ciliated n1bercles, ridges, and bands, all ofwhich contain receptors for tactile senses, cover the body wall.ExcretionAnnelids excrete ammonia, and because ammonia diffusesreadily into the water, most nitrogen excretion probablyoccurs across the body wall. Excret01y organs of annelids are
Annelida: The Metameric Body Formmore active in regulating water and ion balances, althoughthese abilities are limited. Most marine annelids, if presentedwith extremely diluted seawater, cannot survive the osmoticinflux of water and the resulting loss of ions. The evolu tion of efficient osmoregulato1y abilities has allowed only afew polychaetes to invade freshwater. Freshwater annelidsexcrete copious amounts of ve1y dilute urine, although theyretain vital ions, which is important for organisms living inenvironments where water is plentiful but essential ions arelimited. In addition to ammonia, earthworms excrete urea, aless toxic nitrogenous waste.The excretory organs of annelids, like those of manyinvertebrates, are called nephridia. Annelids have two typesof nephridia. A protonephridium consists of a tubule with aclosed bulb at one end and a connection to the outside of thebody at the other end. Protonephridia have a tuft of flagellain their bulbular end that drives fluids through the tubule(figure 12.8a; see also figure 10.6). Some primitive annelidspossess paired, segmentally arranged protonephridia thathave their bulbular end projecting through the anterior sep tum into an adjacent segment and the opposite end openingthrough the body wall at a nephridiopore.Most annelids possess a second kind of nephridium,called a metanephridium. A metanephridium consists ofan open, ciliated funnel, called a nephrostome, that projectsthrough an anterior septum into the coelom of an adjacent seg ment. At the opposite end, a tubule opens through the bodywall at a nephridiopore or, occasionally, through the intestine(figure 12.Sb and c). There is usually one pair of metane phridia per segment, and tubules may be extensively coiled,with one portion dilated into a bladder. A capillary bed is usu ally associated with the tubule of a metanephridium for activetransp01t of ions between the blood and the nephridium(figure 12.Sd; see also figures 12.17 and 12.18).Most annelids have chloragogen tissue associated withthe digestive tract. Chloragogen tissue surrounds the dorsalblood vessel and lies over the dorsal surface of the intes tine (see figure 12.13). Chlorogogen tissue acts similarly tothe vertebrate liver in that it deaminates amino acids and,in earthworms, converts ammonia into urea. It also convertsexcess carbohydrates into energy-storage molecules of glyco gen and fat.Regeneration, Reproduction,and DevelopmentMany annelids have remarkable powers of regeneration.They can replace lost parts, and some species have breakpoints that allow worms to sever themselves when a predatorgrabs them. Lost segments are later regenerated.Some polychaetes reproduce asexually by budding orby transverse fission; however, sexual reproduction is muchmore common. Most polychaetes are dioecious. Gonadsdevelop as masses of gametes and project from the coelo mic peritoneum. Primitively, gonads occur in every body seg ment, but most polychaetes have gonads limited to entralblood vesselNephrostome(d)IExternal openingFIGURE 12.8Annelid Nephridia. (a) Protonephridium. The bulbular endsof this nephridium contain a tuft of flagella that drives wastesto the outside of the body. In primitive annelids, a gonoduct(coelomoduct) carries reproductive products to the outside ofthe body. (b) Metanephridium. An open ciliated funnel (thenephrostome) drives wastes to the outside of the body. (c) Inmodern annelids, the gonocluct and the nepbridial tubules undergova1ying degrees of fusion. Cd) Ne
The Samoan palolo worm is a member of the phylum Annelida (ah-nel' i-dah) (L. annellus, ring). Other members of this phylum include countless marine worms, the soil-building earthworms, and predatory leeches (table 12.1). Characteristics of the phylum Annelida include: 1.