by St V. Apathy, R.
Blanchard, H. Bolsius, A. Dendy, R.S. Bergh, &c. (F. E. B.)

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.)

CHAFER, a word used in modern speech to distinguish the beetles of the
family _Scarabaeidae_, and more especially those species which feed on
leaves in the adult state. The word is derived from the O. Eng.
_