The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta (meaning many setae); the earthworms and fresh-water worms, or Oligochaeta (meaning few setae); the marine and fresh-water leeches or Hirudinea; and two other marine classes having affinities with the Polychaeta: the Archiannelida (meaning primitive annelids), small heteromorphic marine worms, and the Myzostomaria (meaning sucker mouths), parasites of crinoid echinoderms. These five groups share few common characters and little resemblance except that most have a wormlike body. Typically they are bilaterally symmetrical, lack a skeleton, and have a short to long linear body divided into rings or segments, which are separated from one another by transverse walls or septa. The mouth is an anteroventral or anterior vent at the forward end of the alimentary tract, and the anus posterodorsal or posterior at the hind end of the gut. See Hirudinea, Oligochaeta, Polychaeta
The linear series of segments, or metameres, from anterior to posterior ends constitute the annelid body. These segments may be similar throughout, resulting in an annulated cylinder, as in earthworms and Lumbrineris. More frequently the successive segments are dissimilar, resulting in regions modified for particular functions. Each segment may be simple (uniannular) corresponding to a metamere, or it may be divided (multiannulate). The total number of segments varies from five to several hundred. Segments may have lateral fleshy outgrowths called parapodia (meaning side feet), armed with special secreted bristles or rods, called setae and acicula; they provide protection and aid in locomotion. Setae are lacking in Hirudinea and some polychaetes. The body is covered by a thin to thick epithelium which is never shed.
The ability to replace lost parts is highly developed in annelids. Most frequent is the replacement of tail, parapodia, and setae. The anterior end may be replaced provided the break is postpharyngeal. The torn end is first covered over with scar tissue, then differentiated into epithelial cells and all other tissues characteristic of the whole animal.
Depending on the species, reproduction may be sexual, asexual, or both. Sexual reproduction may be dioecious, in which male and female are similar, rarely dissimilar. Individuals may be hermaphroditic, both male and female, but with cross fertilization. Some annelids are protandric hermaphrodites, in which the sexual stages alternate.