ANNELIDA

annelida, a name derived from j.b.p. lamarck's term _annelides_, now used to denote a major phylum or division of coelomate invertebrate animals. annelids are segmented worms, and differ from the arthropoda (q.v.), which they closely resemble in many respects, by the possession of a portion of the coelom traversed by the alimentary canal. in the latter respect, and in the fact that they frequently develop by a metamorphosis, they approach the mollusca (q.v.), but they differ from that group notably in the occurrence of metameric segmentation affecting many of the systems of organs. the body-wall is highly muscular and, except in a few probably specialized cases, possesses chitinous spines, the setae, which are secreted by the ectoderm and are embedded in pits of the skin. they possess a modified anterior end, frequently with special sense organs, forming a head, a segmented nervous system, consisting of a pair of anterior, dorsally-placed ganglia, a ring surrounding the alimentary canal, and a double ventral ganglionated chain, a definite vascular system, an excretory system consisting of nephridia, and paired generative organs formed from the coelomic epithelium. they are divided as follows: (1) haplodrili (q.v.) or archiannelida; (2) chaetopoda (q.v.); (3) myzostomida (q.v.), probably degenerate polychaeta; (4) hirudinea (see chaetopoda and leech); (5) echiuroidea (q.v.). (p. c. m.)