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Which of the following is/are true about polychaete worms?
Introduction to Worms
Most people are familiar with earthworms found in garden soil. Although many different kinds of animals are commonly lumped together as “worms,” there are several distinct phyla that fit the category. Worms are typically long, thin creatures that get around efficiently without legs. The different phyla of worms display a great range in size, complexity, and body structure. Flatworms (phylum Platyhelminthes) are simple animals that are slightly more complex than a cnidarian. Roundworms (phylum Nematoda) have a slightly more complex body plan. Segmented worms (phylum Annelida) are the most complex animals with worm-like body plans. A study of worms can illuminate a possible history of how some organ systems and body features evolved.
Worms are invertebrate animals with bilateral symmetry. Worms have a definite anterior (head) end and a posterior (tail) end. The ventral surface of worms and other organisms is the bottom side of the body, often closest to the ground. The dorsal surface is located on the upper part of the body facing the sky. The lateral surfaces are found on the left and right sides of the body. Figure 3.35 compares bilateral symmetry in a whale shark and a swimming plychaete worm. Organs for sensing light, touch, and smell are concentrated in the heads of worms. They can detect the kinds of environment they encounter by moving in the anterior direction.
There are six features and systems that reveal an evolving complexity in the body structure of most worms:
- a mesoderm, an intermediate body layer between the inner (endoderm) and outer (ectoderm) tissue layers that forms muscle tissue
- a central nervous system guided by a “brain”
- an excretory system to eliminate some kinds of waste products
- a complete digestive system, from an anterior mouth to a posterior anus
- a coelom, a body cavity between the digestive tube and the external body wall that is lined with tissue
- a circulatory system consisting of a series of tubes (vessels) filled with fluid (blood) to transport dissolved nutrients, oxygen, and waste products around the body rapidly and efficiently
Annelid – Locomotion, feeding, and general behavior
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, 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
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
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) 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 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
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.
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 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.
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.
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.
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.
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.
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.
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, 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.) 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.
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