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ANNELIDA

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Originally appearing in Volume V02, Page 73 of the 1911 Encyclopedia Britannica.
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ANNELIDA , a name derived from J. B. P. See also:

Lamarck's See also:term Annelides, now used to denote a See also:major phylum or See also:division of coelomate invertebrate animals. Annelids are segmented See also:worms, and differ from the See also:Arthropoda (q.v.), which they closely resemble in many respects, by the See also:possession of a portion of the coelom traversed by the alimentary See also:canal. In the latter respect, and in the fact that they frequently develop by a See also:metamorphosis, they approach the See also:Mollusca (q.v.), but they differ from that See also:group notably in the occurrence of metameric segmentation affecting many of the systems of See also:organs. The See also:body-See also:wall is highly See also: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 See also:special sense organs, forming a See also:head, a segmented See also:nervous See also:system, consisting of a pair of anterior, dorsally-placed ganglia, a See also:ring surrounding the alimentary canal, and a See also:double ventral ganglionated See also: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) See also:Haplodrili (q.v.) or Archiannelida; (2) See also:Chaetopoda (q.v.); (3) See also:Myzostomida (q.v.), probably degenerate See also:Polychaeta; (4) Hirudinea (see CHAETOPODA and See also:LEECH); (5) See also:Echiuroidea (q.v.). (P. C.

End of Article: ANNELIDA

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