Annelida

Annelida, a name derived from J.B.P. Lamarck’s term Annélides, 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.).