Annelid 
Annelids, or true-segmented worms, are members of the animal phylum An­nelida, the most complex of all wormlike groups of organisms. Annelids are commonly found in terrestrial, as well as marine, brackish, estuarine, and freshwater ecosystems worldwide. Most annelids are free-living, although several species have parasitic, mutualistic, or commensal relationships with other animals, and many species are commonly associated with aquatic and terrestrial plants.
Six major classes comprise this phylum: Polychaeta (polychaete, or many-bristled worms; primarily marine; more than 15,000 species [spp.]); Oligochaeta (oligochaete worms; freshwater, terrestrial, marine; more than 8,000 spp.), Hirudinea (leeches; freshwater, terrestrial, marine; more than 700 spp.), Branchiobdellida (crayfish worms; freshwater, live on crayfishes; more than 100 spp.), Aphanoneura (suction-feeding worms; freshwater; more than 30 spp.), and Acanthobdellida (bristle leech; parasitic on Arctic marine fishes; 1 sp.). As with any group of organisms, the phylogenetic re­lationships of the diverse groups within annelids, and of the phylum to oth­ers within the animal kingdom, is the subject of continuing debate. The marine invertebrate groups Echiura and Sipunculida recently were aligned with the annelids.
All annelids are bilaterally symmetrical; with an elongated, cylindrical body shape divided both externally and internally by a regular, linear series of segments. The highly developed digestive, circulatory, nervous, and ex­cretory systems within the body cavity, or coelom, reflect external segmen­tation and generally are repeated serially; this is called metameric segmentation, and distinguishes annelids from all other wormlike groups. Annelids range in size from less than 0.7 millimeters (0.019 inch) to over 3 meters (9.8 feet) in length. The number of segments is relatively fixed in some groups (Branchiobdellida, Hirudinea), but indeterminate in others. Ex­ternal form of annelids is diverse, even within each group; the polychaetes may have distinct body regions, with limblike parapodia, chaetae (hairs), ten­tacles, and antennae, while others may appear similar to an earthworm, with few if any external appendages. Most oligochaete species have chaetae arranged in bundles on each segment. Several aquatic oligochaetes and many polychaetes have gills.
Leeches are usually flattened, with a posterior sucker and anterior suck­erlike mouth; several species have jaws, others have an extendable proboscis. The branchiobdellidans have a posterior sucker and an anterior end with several fused segments and distinct teeth. Chaetae are absent in leeches and branchiobdellidans. The single species of Acanthobdellida is shaped like an elongate leech, with a few hooked chaetae located ventrally on a few ante­rior segments. Annelids are hermaphroditic; reproduction is commonly sex­ual, but many species reproduce asexually by budding or fragmentation. Annelids are important components of their respective habitats, whether it is the bottom of freshwater or marine environments, or the soil. The feed­ing habits of many species are important in the decomposition of organic matter and recycling of nutrients in terrestrial and aquatic environments. Many annelids feed on algae, insects, carrion, and other worms, and several leech species consume the blood of turtles, birds, fishes, and mammals.
 
References
Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer As­sociates, Inc., 1990.
Dindal, Daniel L., ed. Soil Biology Guide. New York: John Wiley & Sons, 1990.
Fauchald, Kristian. “Worms, Annelida.” In Encyclopedia of Biodiversity, Vol. 5. Edited by S. A. Levin. San Diego, CA: Academic Press, 2001.
Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: Saunders College Publishing, 1994.
Thorp, James H., and Alan P. Covich. Ecology and Classification of North American Freshwater Invertebrates, 2nd ed. San Diego, CA: Academic Press, 2001.