Chaetopoda

Chaetopoda is a great group of Round Worms, which includes those which bear setae. They are composed of many similar segments, with a head or "prostomium." The nervous system consists of a pair of cerebral ganglia, and a paired cord typically running along the middle of the under side. The excretory organs or nephridia are repeated in a series of pairs, usually one in each segment of the body; but the number may be limited as to six in the "lob worm" (Arenicola piscatorum), or there may be many in each somite as in Acanthodrilus multiporus. The generative organs may or may not be repeated: the worms are either hermaphrodite, as the "Earthworm" and most Oligochaeta, or there may be two separate sexes as in most Polychaeta. The setae are the most characteristic structures: they may be very conspicuous, as in many Polychaeta, in some of which, as in Aphrodite, the "sea mouse," they are woven into an iridescent felt; or they may be small and few, as in the Earthworm, or even absent, as in the genera AElosoma and Anachaeta. In the Polychaeta they are borne on two lateral processes known as "parapodia:" these are composed of two lobes, a dorsal, or "notopodium," and a ventral or "neuropodium." Each of these may also bear a series of respiratory tufted branches, or "cirri." In some cases the cirri may be simple, and flattened out into scale-like "elytra" which protect the exposed dorsal surface, as in Polynoe; these may, however, retain their respiratory function as well as being protective. In most cases the anus is terminal, but in a few there are some small postanal somites, as in Criodrilus. Many forms are armed with teeth, as in some species of the family Syllidae. These are perforated by a poison canal. As to mode of life they are either terrestrial, as most Oligochaeta, e.g. the earthworm, or aquatic, whether marine as most Polychaeta or freshwater as Tubifex: some are free-swimming, as the Errantia, others live in tubes as Serpula, or burrow through mud and sandbanks as Arenicola. The only freshwater Polychaeta are a few species of Lumbricones. A few are parasitic, such as Oligonathus bonelliae in some Gephyreans. The Chaetopoda are divided into four orders, the Oligochaeta, Polychaeta, Archiannelida, and Archichaetopoda.