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Characteristics, distribution and ecology
Taxonomische classification
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(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM. Copyright 2002 ETI / Freeman & Co Publishers)

Annelid worms—polychaetes, earthworms (oligochaetes), and leeches (hirudineans)—are distinguished by linear series of external ringlike segments; the grooves between segments coincide with internal compartments, often separated by transverse sheets of tissue (septa), containing serially repeated nervous, muscle, and excretory systems. Anterior segments bear jaws, eyes, and cirri (singular: cirrus, a slender appendage) in some species; the terminal segment may bear a cirrus. Annelids have spacious, mesoderm-lined coeloms—except for leeches, in which tissue packs the coelom—and their coeloms are important in excretion, circulation, and reproduction. Chitinous lateral bristles called setae on each segment are used for locomotion or to anchor the annelid in substrate or burrow; leeches lack setae. Parapodia are unique to polychaetes; these thin, fleshy flaps protrude laterally from each body segment. Chitinous cuticle covers the entire body.
Annelids live in soil, fresh water, and oceans—including Antarctic seas. They may be striped or spotted, and pink, brown, or purple. Others are iridescent or luminescent. Some have colorful gills and cirri, modified parapodia. The endangered Australian earthworm Megascolides, which is 3 m in length, is the largest species; the smallest annelid is only 0.5 mm.
Active predation or scavenging is the feeding mode for most annelids. Many annelids burrow incessantly, turning over and exposing detritus and soil and aerating anaerobic muds and sands; these activities are known as bioturbation. Swimming annelids catch fish eggs or larvae. Filter-feeding marine annelids capture bacteria and feed selectively on sediment particles within tubes (which they build of mucus-cemented sand grains, calcium carbonate, protein and polysaccharide compounds, and other materials) buried in sand or mud. Some trap plankton on a mucus-covered, ciliated eversible proboscis. Others pop out of their tubes to seize prey. Certain species harvest algae growing on their tubes. The sea star Luidia (Phylum Echinodermata), hosts the polychaete Podarke among its tube feet. Some carnivorous polychaetes have fangs with which they inject toxins into prey.
About 15,000 species of annelids are grouped into three classes: the Polychaeta, mostly marine and a few soil and freshwater bristle worms; the Oligochaeta, terrestrial and freshwater bristle worms; and the Hirudinea, or leeches. There are about 9000 species of polychaetes, including myzostomarians, a group of about 100 species of small polychaetes that live on or in echinoderms (Phylum Echinodermata), 6,000 species of oligochaetes, and 500 species of hirudinids.
In most polychaetes (paddle-footed worms), a fleshy lobe (prostomium) projects over the ventral mouth and bears tentacles. Parapodia of all polychaetes with few exceptions are stiffened by a bundle of chitinous bristles, enabling the parapodia to function as oars and levers. In a few polychaete species, chitinous rods called acicula support parapodia. Some tube-dwelling polychaetes leave their tubes, others do not; all of these tube builders are grouped as sedentary polychaetes. Free-living polychaete species are grouped as errant polychaetes. Marine polychaetes include Aphrodite, the hairy sea mice; lugworms (Arenicola), which burrow in sand and mudflats; sabellid and serpulid worms, whose tubes encrust shells, rocks, and algae, including peacock worms (Sabella), which construct mosaic tubes of sand or shell; economically important bait worms such as Nereis; and a few pelagic (ocean dwelling) species. The concretelike tubes of some polychaetes foul ships. There are also some soil and freshwater (in the Great Lakes and Lake Baikal) polychaete species. Oligochaetes include the earthworms and a few small freshwater, estuarine, and recently described deep-sea forms. The hirudinids, with anterior and posterior suckers, are popularly called leeches. Most leeches are free-living predators of frogs, turtles, fish, birds, and invertebrates of soil, foliage, algal thalli, fresh and salt water; a few parasitize vertebrates and invertebrates. Oligochaetes and leeches usually lack distinct eyes, tentacles, and parapodia including gills. Light-sensitive cells and sensory hairs in the earthworm epidermis that connect to the central nervous system alert earthworms to light and other environmental stimuli. Leeches lack setae. Annelids, except for the leeches, regenerate lost body parts from bristles to terminal body segments. Some polychaetes reproduce by budding.
Bloodletting by the freshwater leech Hirudo medicinalis is used to control swelling subsequent to the reattachment of severed fingers or transplanted tissue. Saliva of bloodsucking leeches contains the anticoagulant hirudin as well as an anesthetic. This leech harbors a symbiotic bacterium Aeromonas hydrophila; not only does the bacterium digest blood, it also produces an antibiotic that kills other bacteria.
Burrowing polychaetes turn over 1900 tons of seafloor sand per acre each year. Charles Darwin calculated that earthworms bring 18 tons of soil to the surface per acre each year. Suction by a muscular pharynx draws soil into the earthworm mouth. Food is passed through the esophagus by the peristaltic movements of digestive tract muscles to a crop, where the food is temporarily stored. The muscular gizzard of the earthworm grinds seeds, eggs, larvae, small animals, and plants ingested with soil. Annelids’ longitudinal and circular body-wall muscles work against the coelomic fluid, which is—like all fluids—relatively incompressible; this system functions as a hydraulic skeleton. Thin peritoneal sheets called septa separate adjacent segments. As body-wall muscles contract, colorless coelomic fluid flows from segment to segment through openings in each septum. Food is pushed through the gut from mouth to anus by cilia or by peristaltic contractions of muscles that encircle the digestive tract. The aquatic annelid pumps water through its burrow with peristaltic body waves, cilia, and parapodia. The water current brings in food and dissolved oxygen and removes waste.
Three iron-containing pigments—hemerythrin, hemoglobin, and the green pigment chlorocruorin—transport oxygen in blood vessels and coelomic fluid in most annelids, in corpuscles or in solution in blood. The annelid dorsal blood vessel is contractile with unidirectional valves, forcing blood through five aortic arches (“hearts”) that act as pressure regulators and then into the ventral blood vessel. From the ventral vessel, blood moves to the digestive tube wall, the body wall, and the nephridia. Blood carried to the body wall exchanges oxygen and carbon dioxide through highly vascularized parapodia (in polychaetes)—sometimes modified into gills—and through the moist body wall, even through the protective cuticle. The continuous coelom of the leech lacks septa; most circulatory functions in leeches are carried out by coelomic fluid that is transported within the contractile channels and sinuses of the coelom itself. The closed annelid circulatory system, with contractile heart, blood vessels, and capillaries, is quite unlike the circulation pattern in arthropods.
The annelid excretory system consists of nephridia; within most body segments, ciliated paired tubules—nephridia—draw in wastes from coelomic fluid and then discharge dissolved waste through external pores called nephridiopores. Additional waste moves into nephridia from the nephridial blood vessels. Gametes also exit through nephridiopores in many annelids and through the mouth in a few species. Nephridia also regulate the water content of the coelomic fluid. Castings of intertidal polychaetes are sand, cleaned of organic matter as it passed through the gut and out of the anus.
Segmental ganglia, bilobed cerebral ganglia (brain) and single or paired ventral nerve cords are the main components of the nervous system. Most polychaete annelids have eyes, some with retinas and lenses. Chemoreceptors, touch receptors, vibration receptors, and statocysts (balance organs) concentrated at the head end link to the ventral nerve.
Breeding polychaetes swarm by the millions, their hormones triggered by phases of the moon, the tides, or changes in temperature. Polychaetes are usually dioecious, oligochaetes are usually monoecious but cross-fertilize, and leeches are monoecious. Polychaete gametes arise from the coelom walls in a number of body segments; polychaetes lack permanent gonads. Nephridia usually discharge gametes through nephridiopores in addition to urine, and polychaete fertilization is external. Many polychaete adults brood their young; in some species, the male protects and aerates the eggs. Development of most polychaete annelids includes a free-swimming, ciliated, planktonic larva, the trochophore, which is also often a feeding larva. In some sedentary polychaete species, individuals are budded off or are transformed into epitokes, which are sexually mature, swimming, gamete-bearing individuals. Epitokes of Eunice viridis—the palolo—swarm on the sea surface in the South Pacific, in coincidence with lunar cycles. Male epitokes—stimulated by a pheromone from female epitokes—shed sperm. Responding to sperm, females release eggs. Samoans and other South Pacific people gather palolo epitokes to eat.
In contrast with polychaetes, leeches and oligochaetes are usually hermaphroditic (monoecious); each copulates with another individual. Sperm and eggs are produced in ovaries and testes, rather than in the peritoneal coelom lining. Oligochetes transfer sperm from one worm to its partner, which stores sperm in seminal receptacles until egg laying. The oligochaete lays eggs and releases stored sperm into a secreted mucus band; embryos develop in a secreted cocoon until a juvenile worm escapes. Some leeches attach to their partners with suckers and forcibly drive spermatophores—packets of sperm—into their mates’ bodies; fertilization is internal. Leeches and oligochaetes incubate eggs in a cocoon—an adaptation to terrestrial life. They hatch as miniature adults, without a free-living larval stage.
Those free-living polychaetes that evolved in the Cambrian seas are the most ancient annelid group. From polychaetes, oligochaetes evolved. Leeches are of oligochaete ancestry. Fossil polychaetes have been found in rocks of the late Proterozoic eon that contain the Ediacaran fossil assemblages (about 700 million years old) and are well preserved in the middle Cambrian rocks of the Burgess shale (500 million years old) of western Canada. Terrestrial oligochaetes probably did not evolve before the Cretaceous, when angiosperms, which contributed humus in which earthworms live, arose. Annelids are possible ancestors of sipunculans (Phylum Sipuncula) and echiurans (Phylum Echiura)—at least some species of these phyla have trochophore larvae, produce gametes from peritoneal tissue rather than ovaries and testes, and have similar body-wall anatomy, nervous systems, excretory systems, and patterns of gamete production. Immunological data support annelids as closer relatives of sipunculans than molluscs are; however, the evidence from paleontology, biochemistry, and embryology supports molluscs as closer relatives of sipunculans than are annelids. Data from ribosomal RNA indicate that pogonophorans (Phylum Pogonophora) may have evolved from annelids. Molluscs (Phylum Mollusca), which also have trochophore larvae, may have evolved from annelid ancestors or from sipunculans; alternatively, the trochophore larvae may have arisen independently, an example of convergent evolution. Unlike wormlike nemertines (Phylum Nemertina), nematodes (Phylum Nematoda), nematomorphs (Phylum Nematomorpha), and acanthocephalans (Phylum acanthocephala), annelids have linearly segmented bodies, coeloms, a ventral nerve cord, and distinct eyes (in polychaetes). Onychophorans and annelids may have a common ancestor (see Phylum Onychophora). The phylogenetic relationship between annelids and arthropods is currently controversial.

Literature:
¥ Brinkhurst, R. O., “Evolution in the Annelida.” Canadian Journal of Zoology 60:1043–1059; 1982.
¥ Dales, R. O., Annelids, 2d ed. Hutchinson University Library; London; 1967.
¥ Darwin, C. R., The formation of vegetable mould through the action of worms with observations on their habits. 1881. Reprinted as Darwin on earthworms: The formation of vegetable mould through the action of worms. Bookworm Publications; Russelville, AR; 1976.
¥ Edwards, C. A., and J. R. Lofty, Biology of earthworms, 2d ed. Chapman and Hall; London; 1972.
¥ Eernisse, D. J., J. S. Albert, and F. E. Anderson, “Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralian metazoan morphology.” Systematic Biology 41:305–330; 1992.
¥ Laverack, M. S., The physiology of earthworms. Macmillan; New York; 1963.
¥ Wells, G. P., “Worm autobiographies.” Scientific American 200(6):132–142; June 1959. Annelid behavior patterns.

Phylum Annelida