ANNELID

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 (Polychaeta), which are divided into free-moving and sedentary, or tube-dwelling, forms; the earthworms (Oligochaeta); and the leeches (Hirudinea). ( phylumAnnelida ) , 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. Comprising more than 9,000 species, the annelids are divided into three classes: polychaetes, or marine worms; oligochaetes, or earthworms; and hirudineans, or leeches. The polychaetes, or annelids of the class Polychaeta, number more than 6,000 species and are found worldwide, most often in marine environments. They are divided between free-living and sedentary (tube-dwelling) forms. They range in length from less than an inch to more than 20 feet (6 m). Free-living polychaetes have bodies made up of a head (or prostomium), a segmented trunk, and a tail (or pygidium); sedentary polychaetes have a head (which may be distinct or indistinct), gills (in most species), thoracic and abdominal regions, and a tail. Respiration occurs either through gills or the body wall, and the digestive system usually consists of a mouth, an esophagus, an intestine, and an anus. Parasitism is rare in polychaetes, although commensalism is common among the scale worms (order Phyllodocida). The polychaetes exhibit occasional aggressive behaviour, and some species are capable of producing light (bioluminescence), a trait often related to sexual maturity. Both polychaetes and oligochaetes have shown a rudimentary ability to learn by experience. Polychaetes reproduce sexually, though a few sedentary species are known to reproduce asexually, and hermaphroditism is rare. Reproduction is generally accomplished by the shedding of gametes (sperm and eggs) directly into the water. Fertilized eggs develop into diamond-shaped, free-swimming larvae called trochophores, which gradually come to resemble adults once they settle on a suitable surface. Most species are capable of regenerating lost body parts, particularly tails. The oligochaetes, of the class Oligochaeta, number approximately 3,250 species and are generally found dwelling in the soil, although a few species live in aquatic environments. The range of their length compares with that of the polychaetes; their bodies are made up of a head and a cylindrical trunk, which is divided into as many as 600 segments. Setae project from the bottom, or ventral, side of the body. Respiration takes place through the body wall in most oligochaetes, and their digestive systems usually consist of a mouth, a muscular pharynx, an esophagus, a muscular gizzard, a long intestine, and an anus. Oligochaetes move by extending the anterior (front) end of the body, anchoring the extended part with setae, and then contracting the posterior end of the body. At sexual maturity a saddle-shaped thickening in the centre of the body, called a clitellum, is present. Oligochaetes are primarily hermaphroditic, and copulation usually takes place with a pair of worms in a head-to-tail position, simultaneously exchanging sperm and eggs. The clitellum secretes a cocoon, into which are inserted eggs as it passes over the female pores and sperm as it passes over the male pores. The entire development of the young worm takes place inside this cocoon. Aquatic species are often capable of a form of asexual reproduction, in which the body of the worm fragments into segments that then grow into new worms, and some species can lay self-fertilized eggs or reproduce by parthenogenesis. Regeneration is common. The leeches, of the class Hirudinea, number about 300 species and are generally found in freshwater or humid environments. They are elliptical in shape, with 34 segments each and a sucker at both ends of the body (the larger sucker on the posterior end). They can reach lengths of up to 16 inches (40 cm). Respiration occurs through the skin, and leech digestive systems usually consist of a mouth surrounded by the front sucker, an esophagus, a crop, an intestine, a rectum, and an anus. All leeches feed on blood. All are hermaphroditic, and their reproduction is always sexual. Eggs are deposited in cocoons secreted by the clitellum, and the entire development of the young leech takes place inside this cocoon. The mode of locomotion consists in anchoring of a sucker, contraction of muscles, and then attachment of the other sucker. The annelids are important to humankind in several ways. Polychaetes turn over sediment on the bottom of the ocean, while oligochaetes such as the earthworm (Lumbricus terrestris) turn over terrestrial soil. The bloodworm (Glycera dibranchiata), a polychaete, is used as saltwater fish bait. The sludge worm Tubifex, which is an oligochaete, grows near sewer outlets and is used as an indicator of water pollution, as well as food for tropical fish. Earthworms are used as freshwater fish bait and as humus builders in gardens. Leeches have a long history of medical use, and even today, hirudin, an extract made from leeches, is used as a blood anticoagulant. 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. 263341; R.O. Brinkhurst, Evolution in the Annelida, Canadian Journal of Zoology, 60(5):104359 (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. 387437. 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, 17:193284 (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, 20:173308 (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. Ersus (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 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.