annelid

/an"l id/, n.

1. any segmented worm of the phylum Annelida, including the earthworms, leeches, and various marine forms.

adj.

2. belonging or pertaining to the Annelida. Also, annelidan /euh nel"i dn/.

[1825-35; see ANNELIDA]

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Any member of a phylum (Annelida) of invertebrate animals that possess a body cavity (coelom), movable bristles (setae), and a body divided into segments by crosswise rings.

Known as segmented worms, annelids are divided into three classes: marine worms (Polychaeta; see polychaete), earthworms (Oligochaeta), and leeches (Hirudinea).

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▪ invertebrate

Introduction

phylum name  Annelida,  also called  segmented worm,  

      any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name. The coelom is reduced in leeches, and setae are lacking a few specialized forms, including leeches. A major invertebrate phylum of the animal kingdom, the annelids number more than 9,000 species distributed among three classes: the marine worms (polychaete) (Polychaeta), which are divided into free-moving and sedentary, or tube-dwelling, forms; the earthworms (earthworm) (Oligochaeta (oligochaete)); and the leeches (leech) (Hirudinea).

General features

Distribution and abundance

      Annelids are found worldwide in all types of habitats, especially oceanic waters, fresh waters, and damp soils. Most polychaetes live in the ocean, where they either float, burrow, wander on the bottom, or live in tubes they construct; their colours range from brilliant to dull, and some species can produce light. The feather duster (Manayunkia speciosa) inhabits the Great Lakes and some rivers of the United States. The polychaetes include more than 6,000 known species, which are about evenly divided between free-moving and tube-dwelling forms. The oligochaetes number about 3,250 known species. Oligochaetes, including earthworms, burrow into soil; certain small oligochaetes are found in fresh water, and a few are marine, usually inhabiting estuarial or other shallow waters. Leeches, which number about 300 species, inhabit freshwater or humid environments and are carnivorous or parasitic on other organisms—e.g., all marine leeches are parasitic on fish.

Size range and diversity of structure

      The length of annelids varies from a fraction of an inch to more than six metres (about 20 feet). The width may exceed 2.5 centimetres (about one inch) in the contracted state. Free-moving polychaetes and earthworms include the largest species. Leeches attain lengths of about 0.4 metre in the contracted state.

Polychaetes

 The body of free-moving polychaetes (see figure—>) consists of a head, or prostomium, which may bear two or more eyes; a preoral segment, with such appendages as antennae, tentacles, and palpi (fleshy sensory projections); a trunk divisible into distinct segments; and a tail, or pygidium, which may bear anal cirri (fleshy projections) or plaques and a terminal anus. Each body segment following the second segment (peristome) usually has paired parapodia; i.e., fleshy, lateral outgrowths used in feeding, locomotion, or breathing. The parapodia, generally prominent in free-moving polychaetes, bear bundles of setae, which can be extended, and aciculae (needlelike structures), which are used for support.

 The heads of sedentary polychaetes (see figure—>) may be distinct or indistinct. Forms with a distinct head generally lack head appendages. Branchiae, or gills (gill), which serve for respiration and as food-gathering organs, are well-developed in many of the tube-dwelling forms. Some have tentacles at the anterior (front) end, and gills arise from the dorsal (upper) surface of a few anterior segments. In these species food is gathered by the tentacles and respiration is confined to the gills. The rest of the body is divided into thoracic and abdominal regions. Parapodia, if present, are generally simple lobes; frequently the setae project directly from the body wall. Many sedentary polychaetes construct tubes (tube worm) made from a substance secreted from cells that constitute the epidermis, or skin. Tubes may consist of calcium carbonate, parchment, or mucus, to which sediment adheres. The anus is at the posterior tip. Tube dwellers generally have an external fecal groove along which fecal material passes forward. Eyes are occasionally present on gills, along the sides of the body, or on the pygidium in sedentary forms that do not live in tubes.

Oligochaetes

      The body of oligochaetes is uniformly segmented and has conspicuous segmental lines. The prostomium is usually a simple lobe overhanging the mouth and lacking appendages. The microscopically small eyes are scattered over the body. The clitellum, a saddle-shaped thickening of the body wall, is present at sexual maturity. The anus is at the posterior tip. Setae generally arise from the ventral (lower) surface of the body.

Leeches

      Leeches have 34 segments, and elongation occurs by the subdivision of these segments. Leeches have a small sucker at the anterior end and a large sucker at the posterior end. A clitellum is present in the mid-region during the reproductive period. The poorly developed eyes are paired structures at the anterior end. Setae are absent.

Importance

      Large earthworms, or night crawlers (Lumbricus terrestris), are cultivated and sold as bait for freshwater fishes and as humus builders in gardens. The sludge worm Tubifex, abundant near sewer outlets and thus an indicator of water pollution, is collected and sold as food for tropical fish. Polychaetes play an important role in turning over sediment on the ocean bottom.

      The medicinal use of leeches, which dates from antiquity, reached its peak in the first half of the 19th century. The European species Hirudo medicinalis formerly was exported throughout the world, and native species also were used. Hirudin, an extract from leeches, is used as a blood anticoagulant.

      The estuarine flats of Maine and Nova Scotia are the principal sources of the bloodworm (Glycera dibranchiata), which is used as bait for saltwater fishes. Reproductive parts of the palolo (palolo worm) (Palola siciliensis), which break off and are found in great numbers during the reproductive period, are used as food in Samoa in the south Pacific.

Natural history

Life cycle

Reproduction (reproductive system, animal)

      In the polychaetes, sexes are usually separate but cannot be distinguished in the immature state until gametes (eggs and sperm) appear. Gametes are derived from the mesodermal linings around the digestive tract. The developing gametes are shed into the coelom, where they are nourished by nurse cells (eleocytes). The gametes, especially eggs, are nourished by the breakdown products of muscle tissue, which are passed on to the gametes via the eleocytes. Ripe eggs and motile sperm may leave the body through gonoducts, or tubes for the passage of reproductive cells; through excretory, or nephridial, pores; or through ruptures of the body wall.

      Most polychaetes shed their gametes into the water. Various major body changes may precede the emission of gametes, the two most profound being epitoky (maturation into a modified, fertile form) and stolonization (the development of stemlike growths). In species of Nereis (rag worm), morphological changes include enlargement of the eyes, enlargement of a specific number of parapodia, replacement and alteration of setae, and development of an anal organ (rosette) for the emission of sperm. Morphological changes occur in species of Syllis as well, but they involve only the part that is shed in stolonization. At sexual maturity these polychaetes leave their burrows and swim in groups before releasing gametes.

      The removal of the brain of a nereid that normally undergoes epitoky causes morphological changes without the subsequent formation of gametes. Nereids that normally do not undergo epitoky are unaffected by the removal of the brain. This suggests that apparently the hormone which stimulates epitoky is present only in species that normally undergo this phenomenon. In syllids, stolonization may produce one or more stolons, or stems, containing developing gametes; epitoky is controlled by a nerve ganglion in the proventriculus part of the digestive tract. Epitokous females produce a pheromone that stimulates the male to spawn. The presence of the sperm in the water initiates spawning in the female. Swarming in certain epitokous species coincides with a specific phase of the Moon, but the causes of such behaviour are unknown. Palolos of the Pacific, for example, swarm on a specific day at certain places each year, an event that can be predicted with precision.

      Another type of epitoky occurs in some sedentary polychaetes; in these worms, the parapodial lobes develop long, thin setae. Either the entire animal or only the posterior portion (e.g., the palolo) leaves the tube or burrow and swims before releasing gametes.

       hermaphroditism—that is, the production of eggs and sperm in one individual—rarely occurs in polychaetes. The free-moving polychaete Ophryotrocha, however, shows marked sexual variability; individual males or females may exist in association with hermaphroditic forms. Experiments with Ophryotrocha suggest that the age at which transition from one sex to another occurs may differ among groups within certain species.

      Asexual reproduction is known in a few sedentary polychaete species. In some genera—Ctenodrilus, Pygospio, and Sabella—fragmentation of the body occurs, sometimes forming single segments, from which new individuals can develop.

      Reproduction in oligochaetes is primarily hermaphroditic; the number, arrangement, and location of the male and female gonads and their pores vary considerably among the various species. Lower oligochaetes (Microdrili) have one pair of testes and one pair of ovaries in successive segments. Higher oligochaetes (Megadrili) retain the two pairs of testes in segments 10 and 11 and the pair of ovaries in segment 13. Developing sperm are frequently stored in seminal vesicles before transfer to female receptacles. Sperm ducts lead from the seminal vesicles to male pores located one or more segments behind the testes. The ovaries are simple outpouchings (ovisacs), with oviducts leading to female pores in the next posterior segment.

      Copulation in oligochaetes is reciprocal—that is, both sperm and eggs are exchanged—and takes place in a head to tail position, with the two ventral surfaces in contact. In lower oligochaetes, the male pores of one worm and the female pores of another are opposite each other, and sperm pass directly from the male pores into the seminal receptacle of the female. Cells associated with the brain secrete a hormone that stimulates gamete development. The worms separate after the gametes have been exchanged.

      The clitellum of the earthworm secretes a case, or cocoon, into which is secreted a material that serves as nourishment for the young and a mucous substance that aids in copulation. The cocoon slips forward and receives eggs as it passes the female pores and sperm as it passes the male pores. Fertilization, therefore, takes place within the cocoon. The cocoon slides over the peristome, becoming completely sealed as it does so.

      Asexual reproduction is common in aquatic oligochaetes; indeed, sexual reproduction is virtually unknown in certain naidid species. Some oligochaetes divide to form a chain of two or more individuals that later break off as young worms. In many genera, individuals lay self-fertilized eggs capable of development. Others exhibit parthenogenesis—the production of young without fertilization—a phenomenon associated with polyploidy (multiple sets of chromosomes) in earthworms and accompanied by degeneration of male gonads.

      All leeches are hermaphroditic, and reproduction is always sexual. The testes, from four to 10 pairs, are arranged by segments, beginning with segment 12 or 13. The testes on each side of the body are connected with the vas deferens, a duct that leads indirectly to the male pore. The female reproductive system consists of one pair of ovisacs containing the ovaries, which, although located in front of the testes, may extend some length posteriorly, depending on the animal. The ovaries connect to form an oviduct that forms either a female pore or, in those species that copulate, a vagina.

      In one leech family (Gnathobdellae), sperm are transferred by the penis of one animal into the vagina of another. In two other families (Rhynchobdellae and Erpobdellidae), sperm are transferred by sperm capsules, or spermatophores, onto the body of the leech, after which the sperm leave the spermatophore and enter the ovary through the female pore to unite with the eggs. Leech eggs, numbering from one to more than 100, are usually deposited in cocoons, which may be oval or elongated in shape and are generally attached to rocks or vegetation. Glossiphoniids produce a membranous cocoon and attach it to their ventral surface, where development takes place. A clitellum, which forms only during the reproductive period, secretes the cocoon and material (albumin) to nourish the developing young.

Development

      Annelid eggs, like those of flatworms and mollusks, exhibit spiral, or determinate, cleavage, so called because early differentiation of various regions occurs; in indeterminate cleavage (in echinoderm and chordate eggs), early differentiation does not occur.

      In annelids, the first four cells (blastomeres) give rise, by alternating clockwise and counterclockwise divisions, to a cap of smaller cells, called micromeres, at one end of the egg and a cap of larger cells, called macromeres, at the other end.

      All cells divide simultaneously during the early stages of annelid development; during later stages, however, macromeres divide more slowly than micromeres. As a result, a ball of cells (solid gastrula) forms as the micromeres grow over the macromeres. The gastrula may form by invagination (infolding of cells), epiboly (overgrowth and lengthening), or by both processes. Some of the micromeres become arranged in a characteristic pattern known as the annelid cross.

      A free-swimming immature form called the trochophore larva develops in the polychaete annelids and during the development of certain other invertebrate groups—mollusks, sipunculids, and lophophores. The trochophore larva of polychaetes is typically diamond-shaped with a circle of short, hairlike projections (cilia), called the prototroch, around the thickest part of the body. Cells bearing the prototroch develop from specific micromeres at the 16-cell stage. The four cells of the annelid cross frequently give rise to a so-called apical tuft of cilia at the anterior end. A tuft of cilia (the telotroch) may appear later at the posterior end.

      The upper half of the trochophore—that portion above or in front of the prototroch—will become the prostomium (head) containing the brain, the eyes, and the prostomial appendages, if present in the adult polychaete. The lower half of the trochophore contains the digestive tract, excretory organs, and other internal organs; it is also the site of future segmentation. The mouth and anus form during the trochophore stage, but the digestive tract may or may not be functional at that point.

      Typically, the first three segments form almost simultaneously in the lower half of the trochophore. Development of the parapodial lobe and the appearance of larval bristles (setae) follow shortly thereafter. The body grows in length by the addition of new segments from the preanal segment (pygidium or tail), which is the site of all additional segment formation. The setae generally fall off the first segment, which becomes the adult peristome, or first postoral segment. The parapodial lobes of this segment either develop peristomial appendages or atrophy, depending upon the polychaete species. Adult setae gradually replace larval setae on the second and third segments.

      Although the features of the trochophore larva are relatively uniform among species within an order, the nature of the larva also depends upon other factors—e.g., egg size and larval ecology. Species lacking a pelagic trochophore stage show special adaptive features—e.g., protection by a parent, formation of an egg capsule, the discharge of eggs within one of the parent's tubes, or viviparity (live birth rather than hatching from eggs). The young of species with a short pelagic larval life—a few days or less—either are protected by a parent throughout much of larval life or are hatched from small eggs, with little or no yolk. The most common polychaete trochophore feeds and has a long pelagic life; food consists of microscopic organisms such as diatoms or dinoflagellates. Structural modifications, usually large numbers of setae or bands of cilia around each newly formed segment, facilitate the long pelagic life. After settling, the young polychaete quickly loses its trochophore characteristics and begins to resemble the adult.

      Development in oligochaetes takes place entirely within the cocoon; (cocoon) there is no free-living larval stage. The cocoon of the aquatic lower oligochaetes contains large eggs and relatively little albumin. The cocoon of the terrestrial higher oligochaetes contains small eggs but large amounts of albumin, which nourishes the developing embryos. The oligochaetes undergo a highly modified form of spiral cleavage. The ectoderm, endoderm, and mesoderm, however, arise in the same manner as in the polychaetes. The elongated gastrula has a ventral mouth at the front end and a posterior anus. At the gastrula stage, earthworms begin to feed on the albumin, the embryo elongates, and the mesoderm bands break into units to form the walls, or septa, of individual segments; the worm then leaves the cocoon and begins to construct a burrow nearby.

      Early development in the leeches is similar to that of the oligochaetes. The mesodermal bands form the individual segments as in the oligochaetes, beginning anteriorly. As these mesodermal bands hollow out to form the coelom, mesenchymal cells from the lining of the coelom begin to form one large coelomic cavity extending the entire length of the worm; as the number of mesenchymal cells increases, however, the coelom becomes filled with them. This is the characteristic state of adults. Young leeches hatch from cocoons after feeding upon albumin.

Life span

      Since both the polychaetes and oligochaetes are able to regrow lost parts—i.e., regenerate (see below)—it may appear that they are essentially ageless. Few longevity studies have been carried out with polychaetes, however. Most of the adults of species studied have a characteristic number of segments, which form rapidly during early life and prior to the appearance of gametes. Many polychaetes, especially among the nereids, reproduce only once and then die. In nature these worms, usually quite sluggish after spawning, are eaten by fish and other animals. Species of polychaetes are known to live from one month (Dinophilus) to three years (Perinereis). Species that form stolons (stems), such as the syllids, or whose posterior end breaks off, such as the palolo, are capable of repeating the process; but the number of times and the length of time they are able to do so have not been established. Most sedentary polychaetes survive following spawning, but, again, it is not yet known how often this process can be repeated.

      The life-span of oligochaetes is better established because they are frequently used in laboratory experiments. Asexual reproduction for 130 generations has been reported in one aquatic species. Some earthworms are believed to live as long as 10 years. Senescence, or aging, is known to occur in oligochaetes; Eisenia, for example, lives beyond a reproductive period with a progressive loss of weight. Aging oligochaetes darken in colour, largely as a result of an increase in pigment deposition. In addition, the metabolic rates decrease, and their physiological processes slow.

      Little is known about the life-span of leeches. One species of Erpobdella requires a year to reach sexual maturity, after which it lays cocoons once and dies. Another species breeds once a year for two years and dies during the third.

regeneration

      It has been said that annelids are the most highly organized animals with the power of complete regeneration. The powers of regeneration are greater in the polychaetes and lower oligochaetes than in the higher oligochaetes; leeches lack the ability to regenerate. Most polychaetes and oligochaetes can regenerate a new tail. The ability to replace an amputated part is usually restricted to the anterior end, where lost segments are replaced either by the same number or fewer; if fewer segments form, internal reorganization of the organ system follows. Regeneration from a single segment occurs naturally in the polychaetes Ctenodrilus and Dodecaceria.

      The process of regeneration occurs in a series of steps. First the wound seals over; then a structure (blastema) forms on the surface of the wound. New tissue probably arises from preexisting parent tissue, although mesodermal regenerative cells known as neoblasts, which migrate to the site of the injury, are found in polychaetes and lower oligochaetes. As healing begins, RNA (ribonucleic acid) accumulates at the wound site, first in the epidermal cells and later in mesodermal cells. The amount of glycogen, a complex carbohydrate that serves as an energy source in animals, in the oligochaete Eisenia decreases markedly near the point of injury, returning to normal only after regeneration is complete.

      There is evidence that specific hormones control regeneration in both polychaetes and oligochaetes. A hormone from the posterior part of the brain is essential for posterior regeneration; its presence is apparent only after the second or third day following injury. A mature Nereis is unable to regenerate unless brains from young worms with tails removed are implanted in its coelom.

      Reversal of anterior–posterior polarity has been obtained in an earthworm (Perionyx excavatus). A piece removed from the anterior end regenerates a head at both cut ends if the cuts are made simultaneously. If the new anterior head then is removed, the posterior head becomes dominant and evokes tail regeneration at the surface from which the new anterior head was removed.

Ecology

      There are no marine habitats containing specific polychaetes as there are for mollusks and echinoderms. Many species, such as Neanthes arenaceodentata and Capitella capitata, cosmopolitan in distribution, are found throughout the world. Aquatic oligochaete species are widespread in suitable environments; terrestrial forms are less widely distributed, except for the earthworm and others that have been transported to new habitats, generally inadvertently, by humans. The distribution of leeches is similar to that of oligochaetes, with the aquatic forms more widely distributed.

      Some oligochaete species can secrete a tough mucous covering to protect themselves against either summer heat or winter cold. Some terrestrial burrowing forms burrow deeper into the ground during periods of adverse conditions. Some aquatic leeches burrow deep into the bottom of a pond or stream during the warm months. Polychaetes have no known mechanism for adapting to adverse conditions.

locomotion

      The basic features of locomotion in annelids are most easily observed in the earthworm because it lacks appendages and parapodia. Movement involves extending the body, anchoring it to a surface with setae, and contracting body muscles. When the worm begins a forward movement, circular muscles at the anterior end contract, extending the head forward. At the same time the anterior end lifts from the surface to facilitate forward movement. A wavelike contraction originating in the circulatory muscles then passes toward the posterior end. When the wave of contraction nears the mid-region of the body, longitudinal muscles contract, thereby shortening the region. A wave of contraction of longitudinal muscles follows, and the cycle is repeated. The setae of a segment are extended by certain body muscles to prevent backward movement of the segment during the contraction of the longitudinal muscles. The setae are retracted during the circular contraction period. Muscular movement is aided by the compartmentalization of the segment—coelomic fluid, confined by the segment walls, provides a substance against which the muscles can work. The earthworm is capable of reversing the direction of its movement; the waves of contraction pass forward.

      Locomotion in free-moving polychaetes is accomplished by circular, longitudinal, and parapodial muscles and by coelomic fluid. When a worm such as Nereis moves slowly, the contractual force comes from the sweeping movement of the parapodia. The parapodia of each segment are not aligned, and the effective stroke is the backward one, in which the aciculae (needlelike processes) are projected beyond the parapodium and come in contact with the crawling surface. In the recovery, or forward, stroke, the aciculae retract, and the parapodium lifts free of the surface. When a parapodium ends its backward stroke, the next parapodium initiates one. Body undulations, which help the worm to move rapidly, are produced by the contraction of longitudinal muscles stimulated by the backward stroke of parapodium of a particular segment.

      Locomotion in the burrowing polychaetes, especially those forms lacking anterior appendages, is similar to that of the earthworm. In tube-dwelling sedentary forms, such as the Sabellidae (feather-duster worm), locomotion is restricted to movement within the tube. In this group, the parapodia are reduced or absent; specialized setae, the uncini, function in much the same way as do parapodia in free-moving forms.

      Locomotion in the leech may be compared, in part, to that of the inchworm (immature members of the moth family Geometridae); the anterior and posterior suckers serve as points of contact. When the posterior sucker attaches to a surface, the circular muscles contract, beginning at the posterior end. The leech thus elongates and the anterior sucker fastens to the surface. When the posterior sucker is released, a wave of contraction of the longitudinal muscles moves in a forward direction; this completes one cycle. During swimming, the dorsoventral muscles maintain a contracted state, and undulations of the body are produced by waves of contraction of the longitudinal muscles.

Food and feeding (feeding behaviour)

      The nature of the food and feeding methods of the polychaetes is closely related to the structure of the species, particularly of the anterior end. Those species that feed on large particulate matter have a pharynx either with jaws (Glycera) or without (Phyllodoce); both types can be either herbivorous or carnivorous feeders. Those species that feed on fine particulate matter may be filter feeders, surface-deposit feeders, or burrowers. Filter feeders either capture floating material with ciliated tentacles (Sabella) or pump water through their burrows and capture the fine material on a mucous secretion, upon which they feed (Chaetopterus). Surface-deposit feeders may take in material through a pharynx provided with jaws (Neanthes), with an unarmed pharynx (Cirriformia), or with numerous long ciliated tentacles capable of extending one metre or more (Terebella). Burrowers have a structure similar to that of surface-deposit feeders and can be related species. Pectinaria lives with its anterior end in the sediments and feeds on fine material with its tentacles.

      The diet and feeding mechanisms in oligochaetes are not as varied as those in polychaetes. Terrestrial oligochaetes, such as the earthworm, are scavengers and feed upon decaying organic material, especially of plant origin. Some aquatic oligochaetes, aside from being scavengers, feed on micro-algae or protozoans and other microscopic animals.

      Leeches are primarily bloodsuckers. The medicinal leech Hirudo feeds principally on mammalian blood, but it also sucks blood from snakes, tortoises, frogs, and fish; when young, it may eat oligochaetes. Feeding is facilitated by the secretion of hirudin. The leech detaches after becoming engorged with blood, and it may not attempt to feed again for up to 18 months. Marine leeches attach to, and feed directly from, the gills of fish. Other leeches are carnivorous and feed on oligochaetes and snails.

Behaviour and associations.

      Various polychaetes (for example, Syllis, Chaetopterus, Cirratulus, Terebella) are bioluminescent—that is, capable of producing light. The phenomenon occurs within the cells of Polynoe; the lower surfaces of some scale worms (Halosydna) have special photocells that produce light when stimulated. Odontosyllis light production is related to sexual maturity and swarming, which is influenced by lunar cycles. The female produces a bright luminescence that attracts the luminescent male; light production decreases in the female following the release of gametes. In the order Chaetopterida, the process, which involves the discharge of a luminescent secretion from certain segments and from the antennae, is under nervous control; in Chaetopterus, light can be produced in the parapodia by stimulating the ventral nerve. The significance of light production in this genus is unknown, however, because it lives in a tube through which light rays cannot pass. When stimulated, some earthworms produce a luminescent slime from the mouth, anus, dorsal pores, or excretory pores; it is possible that the light is produced by bacteria living in the worm. Luminescence is unknown in leeches.

      Polychaetes, especially the tube-dwelling Sabellida, generally respond to changes in light intensity by withdrawing into their tubes.

       aggressive behaviour has been reported in several species of nereids (a group of free-moving polychaetes); they respond to a stimulus by extending the proboscis (feeding organ) to expose the jaws. Neanthes arenaceodentata fights members of its own sex but not those of the opposite sex. The response may be related to spawning since this species does not swarm but lays gametes in the tube of another individual; fighting thus prevents the occupation of one tube by two individuals of the same sex.

      Both polychaetes and oligochaetes can learn to choose between favourable and unfavourable environments. In an experiment earthworms try about 12 times to bring into their burrow a leaf made immobile by attachment to some object; when an unattached leaf is presented to the worm, it turns to it and ignores the immobilized leaf thereafter.

       commensalism, a beneficial relationship between two types of organisms, is common among certain scale worms (Phyllodocida, an order of polychaetes). These worms may be found in the tubes of sedentary polychaetes, in the mantle cavity of mollusks, such as chitons and limpets; and on certain echinoderms, such as the starfishes and in the rectums of sea cucumbers. The scale worm Arctonoe, which normally lives on starfishes, is attracted to water flowing from the host starfish but not to that from other starfish species. It has been established that the attractant in the water is a chemical secreted by the host, but its nature is unknown. Tube-dwelling polychaetes, such as Chaetopterus, may be the host to scale worms, pea crabs, or fish, which eat material carried in by water currents produced by the host. Commensalism occurs in some aquatic oligochaete species. The posterior end of Aspidodrilus, for example, is modified as a large sucker for attachment to other worms.

       parasitism is rare in polychaetes. Myzostomida, an atypical polychaete group, are commensal or parasitic either on the surface of or within echinoderms, primarily the crinoids. Polychaete species that live on the surface feed on fine particles carried to the mouth of the crinoid. Parasites that live within crinoids may be found in the body wall, the coelom, or the digestive tract. Parasitic infestations by polychaetes are frequently severe enough to cause wartlike growths on the surface of the host; such growths have been noted on the surfaces of fossil crinoids of the Paleozoic Era (more than 225,000,000 years ago), indicating that these parasites established themselves early. Some forms, such as Iphitime, are parasitic in the branchial chamber of crabs. The young stages of the cosmopolitan polychaete species Arabella iricolor develop in the coelom of species of another polychaete (Diopatra). Some aquatic oligochaetes live in the ureters of toads or in the eyes of frogs. All members of the order Branchiobdellida are parasitic in the brood chambers of the crustacean isopods or on the gills of crayfish, where they suck blood. Many leeches, all of which feed on blood, attach to the host only during feeding. Marine leeches, however, attach permanently to their fish host.

Form and function

External features

      The body of an annelid is often described as a tube within a tube. The inner tube, or digestive tract, is separated from the outer tube, or body wall, by the coelom. The head region (prostomium) is followed by a series of segments similar to each other in appearance. The body in many species, especially in the sedentary polychaetes, is separated into two or three regions. The cells constituting the epidermis (outermost cell layer) are usually simple columnar epithelial cells covered by a cuticle; parts of the body may be ciliated, especially in smaller forms. The cuticle consists of thin layers of protein similar in composition to that of the collagen found in some vertebrate tissues.

 The body form of polychaetes (see figure—>) varies, depending on whether the polychaete is free-moving, sedentary, or pelagic (ocean-dwelling). The first segment, the prostomium, is in front of the mouth and may be a simple lobe or a highly developed projection. The next segment, the peristome, surrounds the mouth and is followed by a series of segments, the total number of which may be limited or unlimited. The parapodia, fleshy outgrowths on each segment following the peristome, contain bundles of setae (movable bristles), which differ in structure and function among species and thus provide a key to species identification. A seta consists of a basal portion within a follicle and a shaft projecting from the follicle; it is secreted from an epidermal cell, which encloses both the ciliary apparatus from which the seta arises and the lacuna in which the seta develops and through which it pushes to the outside. Composite, or pointed, setae are formed from two or more epidermal cells. New setae form in reserve follicles and move forward to replace old ones, which are discarded.

      Branchiae, or gills, are not found in polychaete species that breathe through the body wall. When present, they are simple filaments or tufts near the anterior end of the worm. A mass of feeding structures in sabellid (feather-duster worm) and serpulid polychaete worms, called a tentacular crown, functions both for food gathering and for respiration. Polychaete sensory receptors include eyes, lateral organs, dorsal ciliated ridges, statocysts (organs of balance), taste buds, papillae (blunt-shaped projections), and stiff hairs. The eyes, which range in complexity from simple pigment spots to eyes with lenses, may be found on the prostomium, on the peristome, on the pygidium, along the sides of the body, or on the tentacular crown.

      The oligochaete body is usually cylindrical, is sometimes flattened, and rarely has projecting structures. Segmental lines are usually conspicuous, and secondary segmentation may occur in larger forms. The number of segments varies from seven in some aquatic species to 600 in the earthworms. Setae, embedded in the body wall, may be simple, S-shaped, forked, or hairlike. Except for the first, each segment may have either two pairs of S-shaped setae or a circle of setae. Many transitional forms of setal arrangement occur, and copulatory setae are found on some segments in certain species. All sexually mature oligochaetes have a clitellum (a glandular structure derived from the epithelium), which secretes the egg capsule; it may be saddle-shaped or ring-shaped. In lower oligochaetes it consists of a single layer of modified epithelial cells; in higher forms, such as earthworms, it may have many layers.

      The ends of nerves, which probably respond to touch, heat, and pain, branch among the epidermal cells of oligochaetes. Epithelial sense organs resembling taste buds occur in the skin and mouth cavity; they probably function as chemoreceptors (i.e., smell and taste receptors). Photoreceptors, or light-sensitive organs, are abundant at the anterior and posterior ends of earthworms. Earthworms respond negatively to strong light but are attracted to weak light. All oligochaetes are strongly stereotactic (attracted to surfaces). Some forms have pressure receptors, sensory hairs, and pits.

      A leech, which has 34 segments, may increase in length as a result of subdivision and elongation of the annuli, or rings, that divide each segment. The typical number of annuli per segment in the mid-region is three to five. The anteriorly located eyes usually vary in number from one to five pairs. The clitellum, which is present during reproduction, extends from segments 10 through 12. The most conspicuous of the external features of the leech are the small anterior and the large posterior suckers.

Internal features

Tissues and fluids

 The body cavity of annelids is lined by epithelium. Successive body segments are separated by walls that correspond to the external rings. In grooves between the segments of some oligochaetes are dorsal pores through which coelomic fluid may be discharged. As the leech develops, its coelom becomes nearly filled with connective tissue. Internal features of the polychaetes are shown in the figure—>.

      The coelomic fluid of annelids plays a role in many important functions—e.g., locomotion and regulation of fluid transfer through the body wall (osmoregulation). Many metabolic processes occur in the coelom, which also serves as a site for temporary food storage, for excretion of nitrogen-containing wastes, and for maturation of gametes. The coelomic walls of earthworms contain cells, called chloragocytes, that store and metabolize oil and glycogen and produce ammonia and urea. The chloragocytes eventually disintegrate in the coelomic fluid, and their granules are taken up by amoebocytes, which increase in size, becoming large brown bodies that are never eliminated from the body.

      The fluids of marine polychaetes have the same salt balance as (i.e., are isosmotic with) the surrounding seawater and thus can tolerate no more than a moderate change in the salt (i.e., ion) content of the salt water. Coelomic fluids contain little or no protein. Certain aquatic oligochaetes, however, which live exclusively in fresh water, are capable of regulating the internal medium because, although their coelomic fluid contains fewer salts than does that of polychaetes, it contains more proteins. Freshwater leeches have osmoregulatory (osmoregulation) mechanisms similar to those of oligochaetes.

      The body wall of a typical marine polychaete, such as Perinereis cultrifera, which cannot adapt to salinity fluctuations of seawater, swells and bursts if salinity is reduced to 20 percent that of seawater because the worm has no physiological mechanism for the control of water intake. On the other hand, certain individual Nereis diversicolor worms are capable of tolerating intertidal changes of salinity because they have enlarged nephridia that enable them to excrete excess water.

nervous system

      The nervous system of free-moving polychaetes is similar to that of oligochaetes. It consists of a dorsal brain, or supraesophageal ganglion, which is a discrete mass of nervous tissue in the prostomium; a pair of nerves united ventrally to form the ventral subesophageal ganglion; and paired nerve cords with one ganglion per segment. In sedentary polychaetes, the brain may become highly modified.

      The muscles of annelids are coordinated both by the ventral nerve cord, which is composed of two strands and extends the length of the worm, and by a ganglion and nerves located within each segment. The nerves within each segment carry impulses away from the ganglion (motor nerves) or toward it from a sensory receptor (sensory nerves). The cell bodies of sensory nerves are located beneath the surface epithelium; those of motor nerves are either within the ganglion or in separate parapodial ganglia. Each segmental nerve innervates those components of the body wall, parapodia, and the digestive tract found in its segment.

      The nerve cord of many annelids has giant nerve fibres (neurochords), which may have either a simple or a compound structure. Simple neurochords are very large single nerve cells; their axons arise from cells situated in either the brain or a segmental ganglion. Compound neurochords are multiple structures; each axon is connected to numerous cell bodies along its course. The function of the giant nerve cord is the rapid transmission of impulses from one end of the worm to the other; this enables the longitudinal muscles of each segment to contract at about the same time. The value of rapid contraction is evident in the escape reaction of tube-dwelling sedentary polychaetes.

      Some giant nerve fibres convey impulses as fast as vertebrate nerve fibres (about 21 metres per second); annelid fibres, however, are larger in diameter (1.5 millimetres in Myxicola) and lack a thick insulating sheath (myelin). Not only is recovery from the passage of impulses slower in giant nerve fibres than in other annelid nerves but the former are also the last component to develop in the nerve cord of a growing worm. The nerve cord of Myxicola contains one giant nerve fibre, which is used to study the properties of the nerve impulse. In Myxicola, an impulse may be conducted in either direction along the nerve, unlike Nereis or the earthworm; may be initiated at any level; and is an all-or-none action.

Digestive system (digestive system, invertebrate)

      The polychaete digestive system is generally a straight tube; a mouth leads into an esophagus, which is followed by the intestine and the anus. Some free-moving forms have a proboscis that can be thrust forward by being turned inside out—that is, the proboscis is eversible. In oligochaetes such as the earthworm, the mouth opens into a muscular pharynx, which opens to the esophagus and then to a muscular gizzard. The intestine, which extends most of the length of the worm, terminates in an anus. In leeches, the mouth, surrounded by the anterior sucker, opens into the esophagus; the crop and intestine follow—each with minute pockets (diverticula)—then the rectum and anus.

      Most annelids, except leeches, either lack or have poorly developed diverticula, minute pockets that serve as digestive glands. Instead, the gut lining contains secretory cells (concentrated in the foregut) and absorptive cells (concentrated in the hindgut). Digestive enzymes are most active in the gut. Digestion within cells has not been demonstrated in annelids. A lengthwise fold, the typhlosole, hangs downward in the intestinal cavity of oligochaetes. The absorptive surfaces of the typhlosole and of the anterior intestine may have a brush border; fats are absorbed only in this region.

      Calciferous glands, found only in certain earthworms, apparently excrete calcium by secreting granules of calcium carbonate that are transformed into calcite crystals in the intestine.

Excretory system (renal system)

      The basic units of the annelid excretory system are either protonephridia, which have tubules (solenocytes) that end blindly within cells, contain flagella (whiplike projections), and are joined to a common duct that drains to the outside; or metanephridia, which are funnel-shaped structures containing cilia (short, hairlike processes) that open to the outside.

      Ammonia is the chief nitrogen-containing end product of protein metabolism in aquatic annelids; earthworms, adapted to living in the soil, excrete more of another nitrogen-containing compound, urea, probably as part of a mechanism to control salt and water balance in the worm. The sea mouse Aphrodita, a polychaete, excretes 80 percent of its nitrogen as ammonia, which is also the primary nitrogenous excretory product in leeches (smaller amounts of urea also are excreted). Part of the ammonia excreted by leeches may come from bacteria in part of the leech's excretory system (nephridial capsules). The ability of leeches to withstand high concentrations of ammonia is believed to result from a protective effect provided by high levels of calcium in their cells.

      Three aspects of nephridial function in annelids correspond to those of the vertebrate kidney—filtration, resorption, and secretion. Coelomic fluid filters through solenocytes. The ciliated funnels of metanephridia retain minute particles and those of moderate size. In oligochaetes, whose coelomic fluid contains proteins, particles are actively absorbed in the ciliated region of the tubule. The tubules of earthworms also resorb inorganic ions such as sodium and calcium and can selectively eliminate excretory products from both the coelomic fluid and the bloodstream.

Respiratory system

      Gas exchange generally takes place through the skin, but it may occur through gill filaments in some polychaetes or through the rectum of aquatic oligochaetes. Although oxygen may be transported directly in the blood, it is usually carried by a respiratory pigment, either hemoglobin or chlorocruorin. Hemoglobin, the most common pigment, is present in most free-moving and some sedentary polychaetes and in most oligochaetes and leeches. Chlorocruorin is found in several polychaete groups (Flabelligerida, Terebellomorpha, and Serpulimorpha). A few free-moving polychaetes, some oligochaetes, and rhynchobdellid leeches have colourless blood. The blood of the polychaete Serpula vermicularis contains both pigments, the young having more hemoglobin and the old more chlorocruorin.

      Annelid hemoglobin molecules have several properties in common with the hemoglobin found in vertebrates but differ in molecular weight and in the relative amounts of certain constituents. Chlorocruorin differs from hemoglobin in having a lower affinity for oxygen and in being green in dilute solutions, red in concentrated ones.

      The properties of annelid respiratory pigments are associated with the mode of life of the worm. The hemoglobin of the lugworm Arenicola, a polychaete, releases oxygen to the tissues only under conditions of extreme oxygen deficiency. The hemoglobin of some earthworms takes up oxygen from a normal atmosphere but releases it only when tissue oxygen is low and, thus, may protect the worm from oxygen poisoning.

Circulatory system

      The circulatory system in the lower oligochaetes consists of a dorsal vessel that arises from a blood sinus or capillary network surrounding the intestine and conveys blood forward; a ventral vessel that conveys blood backward; and connective vessels between the two. The blood vessel walls consist of an outer membranous (peritoneal) layer containing muscle fibres, a middle region of collagenous material, and an inner lining of thin cells (endothelium). In higher oligochaetes, one or more pairs of hearts connect the dorsal and ventral vessels and propel the blood. In free-moving polychaetes the dorsal vessel is the chief propulsive force, and networks of small vessels connect the dorsal and ventral ones. In some leeches the blood is propelled by a dorsal vessel connected by loops at both ends to a ventral one.

      Blood is moved by wavelike contractions of the blood vessels, by the beating of cilia, or by pumping provided by hearts. In Arenicola and the earthworm the heartbeat apparently is initiated in nerve cells rather than in muscle tissue, as occurs in vertebrates. The blood apparently carries nitrogen-containing products to the nephridia for excretion. The only blood cells are amoebocytes, which are free-moving cells that engulf particles.

Hormones (hormone)

      The brain contains several types of cells whose secretory activities relate to phases of the life cycle, especially those of reproduction, growth, and regeneration.

      Neurosecretory cells (neurosecretory cell), which are nerve cells that produce hormones, are found in the brain; their structure, similar to that of nonsecretory nerve cells, consists of fine projections (an axon and neurofibrils) and a cell body. The secretions of neurosecretory cells, which terminate in the walls of a blood vessel, in other fluid systems, or in the epidermis, are in the form of microscopic droplets or granules. Neurosecretory cells seem to be derived from epidermal secretory cells that have been incorporated into the central nervous system.

      Inhibitor hormones are known in some Phyllodocida, and a stimulator substance has been identified in Drilomorpha, both of which are polychaete groups. (For a discussion of inhibitor hormones in nereids and syllids, see above Reproduction (annelid).) The maturation of gametes is apparently inhibited in nephtyid polychaetes by neurosecretions of the brain. The brain of the lugworm Arenicola stimulates maturation of gametes.

      The brain has been shown to play a role in the regeneration of the posterior end of the body of polychaetes such as nereids and nephtyids, but the effect may be an indirect one involving the genital inhibiting hormone. Neurosecretory cells occur in the brain and subesophageal ganglia of several terrestrial and aquatic oligochaete species. Removal of the brain from sexually maturing earthworms causes degeneration of the clitellum and prevents gamete formation. The brain also plays a role in osmoregulation, as indicated by the increase in chloride concentration in the urine of oligochaetes lacking a brain. The neurosecretory cells in the brain of leeches control gamete formation.

Evolution and paleontology

      The annelids are considered to have evolved in the sea, perhaps from an ancestral flatworm that evolved through the trochophore larva, the characteristic early stage of polychaetes. The oligochaetes are thought to have developed from polychaete stock; the leeches, which have the clitellum in common with the oligochaetes, probably evolved from the latter.

      The question of which polychaete order preceded the others remains unresolved. The Archiannelida were long considered to have been the earliest polychaete group because of their primitive condition; however, some members (e.g., Polygordius) that lack setae and external segmentation and have simple nervous, muscular, and circulatory systems are now considered to be a specialized group. Polygordius species typically are small in size; they have cilia on their surfaces for locomotion, respire through the skin, and have internal fertilization. Finally, the larvae undergo non-pelagic development. The polychaetes appear therefore to have undergone radiative evolution, in which every character has been modified independently of the others. There is thus little basis for regarding any one order as ancestral to the others.

      The evolution of oligochaetes from polychaetes may be related to the change from a marine to a freshwater habitat. One view is that oligochaetes evolved in marine swamps and were subjected to periodic drying; survival during dry periods would have been made possible by egg cocoons. A contrary hypothesis is that the primitive oligochaete was adapted to permanent freshwater conditions rather than to a terrestrial habitat. Some authorities consider the oligochaetes to have evolved from some members of the order Eunicida (e.g., the family Lumbrineridae) or the order Capitellida (e.g., the family Capitellidae), but this may result from a superficial resemblance in body form and thus may be of little evolutionary significance.

      Reproductive structures provide not only the main criteria for understanding the course of evolution within the oligochaetes, but the basis for the classification of oligochaetes as well.

      Each of the oligochaete orders, Lumbriculida, Monilogastrida, and Haplotaxida, is considered to have evolved separately from primitive oligochaetes. Many, however, believe that two paths of evolution occurred. In one pathway, the vas deferens (the tube carrying sperm from the testes) opened outward on the segment immediately behind the segment that contains the testes and evolved into two lines differentiated on the basis of whether the seminal receptacle (a storage cavity) opened in front of the testes, or at the same segment, or posterior to the testes. In the second principal pathway, the vas deferens opened a few segments behind the testes.

      There is little doubt that the leeches evolved from the primitive oligochaetes, since both groups have a clitellum, at least during the reproductive period, and both are hermaphroditic. The Acanthobdellae are considered to be the link between the oligochaetes and leeches because they possess setae and walls between segments; the order contains only one known species, however. The three remaining orders of leeches evolved into two lines based on whether or not the animals have jaws.

      The fossil record of annelids is limited because they have almost no hard body parts. Tubes constructed by polychaetes and polychaete jaws are the most commonly encountered fossil specimens. Most fossil records of oligochaetes are doubtful, and fossil leeches are unknown. Some burrows, or tubes, have been interpreted as belonging to wormlike creatures from Precambrian strata (more than 620,000,000 years old). Fossils resembling the scale worm Halosydna and the sea mouse Aphrodita, Nereis-like forms, and calcareous tubes similar to present-day Serpula and Spirorbis species have been described. The shells of Paleozoic mollusks (more than 230,000,000 years old) are occasionally marked by U-shaped tubes similar to those made by the polychaete Polydora, a modern-day pest of oysters. The tough jaws of polychaetes, containing minute spiny black teeth known as scolecodonts, occur from the Cambrian Period (about 570,000,000 to 500,000,000 years ago) onward.

Classification

Distinguishing taxonomic features

      Classification of free-living and sedentary polychaetes relies almost exclusively on external characters, such as the shape of the head, and on the number and nature of structures, such as appendages (including anal ones), parapodia, and setae, and on tube construction. Oligochaete classification relies largely on internal structures, especially the arrangement and number of gonads, the position of the gonoducts, and particularly the location of the male pore. Setal characteristics are generally uniform among species. Leech classification is based on the presence or absence of setae and the nature of the mouth, proboscis (feeding organ), jaws, suckers, eyes, and reproductive system.

Annotated classification

      The following classification incorporates the views of several authorities.

Phylum Annelida (annelid) (segmented worms)

 Body wall covered with a cuticle secreted by the epidermis and containing an outer circular and inner longitudinal muscle layer; chitinous (tough, complex carbohydrate material) setae usually present, secreted by follicular cells and arranged segmentally; head or prostomium preoral, with or without appendages; closed circulatory system, with blood often containing a respiratory pigment; coelom, of schizocoelic origin, divided segmentally into compartments by walls, or septa; nervous system includes a dorsal, bilobed brain and a pair of connective nerves that encircle the digestive tract and unite to form a ventral nerve cord with 1 ganglion per segment.

      Class Polychaeta (marine worms (polychaete))

 Paired lateral appendages, or parapodia, bearing chitinous setae; name of group refers to the many setae per segment; head with or without appendages; sexes generally separate with gametes discharged directly into the water, where fertilization and development occur; the free-swimming larva called a trochophore; more than 6,000 living species; free-moving and sedentary (tube-dwelling) forms.

      Order Aphroditamorpha (scale worms (scale worm))

 Free-moving; dorsally rounded, with flattened pairs of scales more or less alternating with the dorsal cirri (slender projections); head with 1 or 3 tentacles, 2 palpi (fleshy sensory projections), and 4 tentacular cirri used for feeding and respiration; projecting (protrusible) proboscis cylindrical in shape, with border of soft papillae (nipplelike projections) and 4 chitinous jaws; size, 0.5 to 25 cm; examples of genera: Aphrodita ( sea mouse), Halosydna (common scale worm), Arctonoe.

      Order Amphinomida

 Free-moving; prostomium with 1 to 5 antennae, 2 palpi, and a caruncle (posterior ridge) deeply set into anterior segments; parapodia with 2 lobes and branchiae (gills); size, 0.5 to 35 cm; examples of genera: Eurythoe (fireworm), Euphrosyne.

      Order Spintherida

 Body oval; median antenna on prostomium; pharynx retractable; dorsal surface with membranous ridges; ventral setae strongly curved; found on sponges; small; single genus, Spinther.

      Order Phyllodocida

 Free-moving; a large group characterized by a protrusible proboscis that may or may not be armed with chitinous jaws, teeth, or papillae; prostomium with 1 to 5 antennae, with palpi, and with 0 to 3 pairs of eyes; parapodia well developed into 1 or 2 lobes usually bearing compound setae; size, 0.2 to over 1 m; examples of genera: Anaitides, Syllis, Hesione, Nereis (rag worm), Glycera (bloodworm), Nephtys, Halosydna.

      Order Eunicida

 Free-moving; head with or without appendages and eyes; proboscis with dorsal maxillae (upper jaws) of 1 to many paired pieces, a ventral pair of mandibles (lower jaws) more or less fused along the median line, and a pair of embedded maxillary carriers; parapodia single-lobed, often with many aciculae (needlelike structures); size, minute to 3 m; examples of genera: Palola (palolo), Eunice, Stauronereis, Lumbineris, Onuphis.

      Order Orbiniida

 Sedentary; head pointed or rounded without appendages; proboscis eversible and unarmed; body divided into distinct thorax and abdomen; gills arise dorsally from thoracic region; size, minute to 40 cm; examples of genera: Scoloplos, Paraonis.

      Order Spionida

 Sedentary; at least 2 long feeding tentacles adapted for grasping and arising from prostomium; size, 0.5 to 25 cm; examples of genera: Spio, Polydora.

      Order Chaetopterida

 Two to 3 distinct body regions; prostomium with palpi; modified setae on segment 4; tube dweller; examples of genera: Chaetopterus ( parchment worm), Spiochaetopterus.

      Order Magelonida

 Long, slender bodies divided into 2 regions; prostomium flattened with 2 long palpi arising from the ventral surface at the junction of the prostomium and next segment; capillary and hooded hooks; single genus, Magelona.

      Order Psammodrilida

 Prostomium and peristome lack appendages; parapodia in mid-region long and supported by aciculae; minute; 2 genera, Psammodrilus and Psammodriloides, each with a single species.

      Order Ctenodrilida

 No prostomial appendages; no parapodial lobes; setae arise directly from body wall; all setae simple; minute; examples of genera: Ctenodrilus, Zeppilina.

      Order Cirratulida

 Sedentary; prostomium pointed and without appendages; 1 or more pairs of tentacular cirri arising from dorsal surface of anterior segments; gills, if present, long and slender, inserted above parapodia; size, minute to 20 cm; examples of genera: Cirratulus, Cirriformia.

      Order Cossurida

 No prostomial appendages; a single median tentacle arises from the dorsum between segments 2 and 6; parapodia biramous with weakly developed lobes; all setae simple; size, usually less than 2 cm; Cossura.

      Order Opheliida

 No prostomial appendages; body with limited number of segments; setae all simple; size, 1 to 10 cm; examples of genera: Ophelia, Polyophthalmus, Scalibregma.

      Order Capitellida

 No prostomial appendages; 1 or 2 anterior segments without setae; parapodia biramous; setae all simple; size, 1 to 20 or more cm; examples of genera: Capitella, Notomastus, Arenicola (lugworm), Maldane, Axiothella.

      Order Flabelligerida

 Sedentary; setae of anterior segments directed forward to form a cephalic (head) cage; prostomium and peristome retractile, with 2 palpi and retractile branchiae; size, 1 to 10 cm; examples of genera: Flabelligera, Stylariodes.

      Order Sternaspida

 Sedentary; anterior setae short and thick; posterior end with ventral shield bearing radiating setae and anal branchiae; size, 3 cm; genera include Sternaspis.

      Order Oweniida

 Sedentary; anterior end with or without divided lobed membrane; anterior segments long; dwelling tube mucoid, coated with sand or shell fragments; size, 0.2 to 10 cm; genera include Owenia.

      Order Terebellida

 Sedentary; head concealed by filamentous tentacles; branchiae, simple or branched, arising from dorsal surface of anterior end; body divided into thorax and abdomen; tube of mucoid substance to which sediment adheres; size, 1 to 40 cm; examples of genera: Amphicteis, Terebella, Pista, Thelepus (tentacle worm).

      Order Sabellida (feather dusters)

 Sedentary; head concealed with featherlike filamentous branchiae; body divided into thorax and abdomen; tube mucoid or calcareous; size, minute to 50 cm; examples of genera: Sabella (peacock worm), Eudistylia, Serpula, Hydroides.

      Order Archiannelida

 Minute, primitive, with ciliated epidermis; prostomium small, with or without appendages; parapodia absent; septa reduced or absent; size, minute. Contains 4 groups of poorly known species considered separate orders by some (Nerillida, Dinophilida, Polygordiida, Protodrilida); genera include Dinophilus and Polygordius.

      Order Myzostomida

 Body disk-shaped or oval without external segmentation; external or internal commensals or parasites of echinoderms, especially crinoids; size, minute to 1 cm; genera include Myzostoma.

      Order Poeobiida

 Body saclike without external segmentation; anterior end with circle of tentacles; 2 internal septa only polychaete characteristics; pelagic; single genus, Poeobius.

      Class Oligochaeta (oligochaete)

 Primarily freshwater or terrestrial with setae arising directly from body wall; name of group refers to the few setae per segment; head and body appendages generally lacking; hermaphroditic, with testes located anteriorly to ovaries; gonoduct system complex; seminal receptacle used to store sperm; reproduction by copulation, with fertilized eggs laid in a cocoon secreted by clitellum; development direct, without larval stages; about 3,250 living species.

      Order Lumbriculida (earthworms)

 Male gonopores several segments behind segments containing the testes or, when 2 pairs of testes are present, in more posterior segment; size, minute to 30–40 cm; examples of genera: Haplotaxis, Eisenia, Lumbricus (earthworm), Megascolides.

      Order Moniligastrida

 Male gonopores, 1 or 2 pairs on segment posterior to testes; clitellum 1 cell thick; 4 pairs of setae per segment; size, minute to 3 m; examples of genera: Moniligaster, Drawida.

      Order Haplotaxida

 Chiefly aquatic worms; male gonopores in segment immediately behind testes; seminal receptacle at or near segment containing testes; size, minute to 1–3 cm; examples of genera: Nais, Tubifex (sludge worm).

      Class Hirudinea (leeches (leech))

 Primarily freshwater, but also terrestrial and marine forms; small sucker at anterior end, large sucker at posterior end; fixed number of body segments at 34; body cavity filled with connective tissue; hermaphroditic, with fertilized eggs laid in a cocoon secreted by clitellum; development direct without larval stages; about 300 living species.

      Order Branchiobdellida

 Head modified as sucker with fingerlike projections; posterior segments also modified to form sucker; body with 14 to 15 segments; all species parasitic or commensal on freshwater crayfish; size, minute; Stephanodrilus.

      Order Acanthobdellida

 Primitive group; setae present on 5 anterior segments; no anterior sucker; parasitic on fish in Lake Baikal (U.S.S.R.); size, small; genera include Acanthobdella.

      Order Rhynchobdellida

 An eversible pharynx used to penetrate host tissue; jawless; distinct blood vessels contain colourless blood; freshwater or marine inhabitants; size, minute to 20 cm; examples of genera: Glossisphonia, Piscicola, Pontobdella.

      Order Arhynchobdellida

 Pharynx with 3 toothed jaws or none, noneversible; terrestrial or freshwater; bloodsuckers or carnivorous; size, minute to 20 cm; examples of genera: Hirudo, Haemopis, Erpobdella.

Critical appraisal

      Most authors accept the annelids as having three major classes: Polychaeta, Oligochaeta, and Hirudinea. Older systems would place the polychaetes and oligochaetes under the class Chaetopoda because both groups possess setae. Other systems would join the oligochaetes and leeches in a single class, called the Clitellata, because both groups possess a clitellum. The Archiannelida and Myzostomida treated as polychaete orders in the classification system above have been considered as separate classes in the past. The Branchiobdellida are considered an order of Hirudinea, but they have been considered as a separate class in the past or as an order of Oligochaeta. Depending upon the author, annelids could consist of as many as six classes.

      Orders were frequently ignored in the past, especially with the polychaetes, but authors have come to greater agreement as to the placement of families within orders. Placement of annelids within orders has been difficult because of the tremendous diversity in structure and specialization in habitat, especially in the polychaetes.

      The class Polychaeta has also been divided into subclasses or orders, the Errantiata (free-moving forms) and Sedentaria (sedentary, or tube-dwelling, forms), based on the mode of living. This arrangement, while convenient, is not based on morphology and is not generally used. The classification system given above lists 23 orders (Archiannelida was considered as one order in the classification above, while other schemes divide the group into four orders). There are approximately 87 known families of polychaetes.

      The oligochaetes are divided into three orders based especially on the placement of the male gonopores. There are approximately 43 families in the class. The families of leeches, organized into the four orders outlined above, are generally accepted.

Additional Reading

General overviews of annelids can be found in R. Phillips Dales, Annelids, 2nd ed. (1967), a semipopular account; D.T. Anderson, Embryology and Phylogeny in Annelids and Arthropods (1973); P.J. Mill (ed.), Physiology of Annelids (1978), a review; Robert D. Barnes, Invertebrate Zoology, 4th ed. (1980), ch. 10, “The Annelids,” pp. 263–341; R.O. Brinkhurst, “Evolution in the Annelida,” Canadian Journal of Zoology, 60(5):1043–59 (1982), a summary of current scholarship; Donald J. Klemm (ed.), A Guide to the Freshwater Annelida (Polychaeta, Naidid and Tubificid Oligochaeta, and Hirudinea) of North America (1985), on ecology and taxonomy; and Vicki Pearse et al., Living Invertebrates (1987), ch. 16, “Annelid Body Plan,” and ch. 17, “A Diversity of Annelids,” pp. 387–437. For information on polychaetes, see Kristian Fauchald, The Polychaete Worms: Definitions and Keys to the Orders, Families, and Genera (1977); Kristian Fauchald and P.A. Jumars, “The Diet of Worms: A Study of Polychaete Feeding Guilds,” Oceanography and Marine Biology, 17:193–284 (1979); and Albrecht Fischer and Hans-Dieter Pfannenstiel (eds.), Polychaete Reproduction: Progress in Comparative Reproductive Biology (1984), a collection of symposium papers. For oligochaetes, see R.O. Brinkhurst and B.G.M. Jamieson, Aquatic Oligochaeta of the World (1971); C.A. Edwards and J.R. Lofty, Biology of Earthworms, 2nd ed. (1977); and O. Giere and O. Pfannkuche, “Biology and Ecology of Marine Oligochaeta: A Review,” Oceanography and Marine Biology, 20:173–308 (1982). See also the proceedings of three international symposia on aquatic oligochaete biology: R.O. Brinkhurst and David G. Cook (eds.), Aquatic Oligochaete Biology (1980); G. Bonomi and C. Erséus (eds.), Aquatic Oligochaeta (1984); and R.O. Brinkhurst and R.J. Diaz (eds.), Aquatic Oligochaeta (1987). For leeches, see Kenneth J. Muller, John G. Nicholls, and Gunther S. Stent (ed.), Neurobiology of the Leech (1981); and Roy T. Sawyer, Leech Biology and Behaviour, 3 vol. (1986), an extensive overview.Donald J. Reish

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