CHAETOSOMATIDA

chaetosomatida, a small group of minute, free-living, aquatic organisms which are usually placed as an annex to the nematoda. indeed mechnikov, to whom we owe much of our knowledge of these forms, calls them "creeping nematoda." they are usually found amongst seaweed in temperate seas, but they are probably widely distributed; some are fresh-water. the genus _chaetosoma_, with the two species _ch. claparedii_ and _ch. ophicephalum_ and the genus _tristicochaeta_, have swollen heads. the third genus _rhabdogaster_ has no such distinct head, though the body may be swollen anteriorly. the mouth is terminal and anterior and surrounded by a ring of spicules or a half-ring of hooks. scattered hairs cover the body. just in front of the anus there is in _chaetosoma_ a double, and in _tristicochaeta_ a triple row of about fifteen stout cylindrical projections upon which the animals creep. the females are a little larger than the males; in _ch. claparedii_ the former attain a length of 1.5 mm., the latter of 1.12 mm. the mouth opens into an oesophagus which passes into an intestine; this opens by a ventral anus situated a little in front of the posterior end. the testis is single, and its duct opens with the anus, and is provided with a couple of spicules. the ovary is double, and the oviducts open by a median ventral pore about the middle of the body; in this region there is a second swelling both in _chaetosoma_ and in _rhabdogaster_. the last-named form is in the female 0.36 mm. in length. in it the hairs are confined to the dorsal middle line and the creeping setae are hooked, of a finer structure than in _chaetosoma_, and situated so far forward that the vagina opens amongst them. _ch. ophicephalum_ has been taken in the english channel. [illustration: from _cambridge natural history_, vol. ii. "worms." by permission of macmillan & co., ltd. mature female of _chaetosoma daparedii_, (from mechnikov.) a, oesophagus; b, intestine; c, anus; d, ovary; e, generative pore; f, ventral bristles.] see e. mechnikov, _zeitschr. wiss. zool._ xvii., 1867, p. 537; panceri, _atti acc. napoli_, vii., 1878, p. 7. (a. e. s.)