Annelida
. Annelid, common name for about 9000 species of wormlike invertebrate animals with well-developed segmentation. The three major classes of the annelid phylum are the bristle worms (about 5300 species), which are mainly marine and often luminescent; the oligochaetes (about 3100 species), which are mostly freshwater or inhabit the soil, such as the earthworm ; and the leeches (about 300 species), which are mainly freshwater but may also be marine or terrestrial. Although annelids are diverse in form, the bristle worms, which generally are considered the ancestral stock, can exemplify their structure. The elongate body is rounded in cross section, with a mouth (sometimes jawed) at one end and an anus at the other, and it exhibits bilateral symmetry . The body is made up of several similar units, or segments, separated externally by furrows and internally by septa (partitions). The segments generally bear parapodia (lobes) with setae (bristles) that are used in locomotion. The coelom (f...