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 I.I2 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 o•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 . See E . Mechnikov, Zeitschr. wiss.- Zool. xvii., 1867, p . 537; Panceri, Atti Acc . Napoli, vii., 1878, p . 7 . (A . E .