w anus subsequently arising. The archenteron gives
off two lateral pounchs and thus becomes trilobed. The middle lobe
forms the alimentary canal; it closes behind and opens to the exterior
anteriorly and so makes the mouth. The two lateral lobes contain the
coelom; each separates off in front a segment which forms the head and
presumably then divides again to form anteriorly the trunk, and
posteriorly the tail regions. An interesting feature of the
development of Chaetognaths is that, as in some insects, the cells
destined to form the reproductive organs are differentiated at a very
early period, being apparent even in the gastrula stage.

The great bulk of the group is pelagic, as the transparent nature of
all their tissues indicates. They move by flexing their bodies.
_Spadella cephaloptera_ is, however, littoral and oviposits on
seaweed, and the "Valdivia" brought home a deep-sea species.

The three genera are differentiated as follows:--

_Sagitta_ M. Slabber, with two pairs of lateral fins. This genus was
named as long ago as 1775.

_Krohnia_ P. Langerhans, with one lateral fin on each side, extending
on to the tail.

_Spadella_ P. Langerhans, with a pair of lateral fins on the tail and
a thickened ectodermic ridge running back on each side from the head
to the anterior end of the fin.

The group is an isolated one and should probably be regarded as a
separate phylum. It has certain histological resemblances with the
Nematoda and certain primitive Annelids, but little stress must be
laid on these. The most that can be said is that the Chaetognaths
begin life with three segments, a feature they share with such
widely-differing groups as the Brachiopoda, the Echinoderma and the
Enteropneusta, and probably Vertebrata generally.

See O. Hertwig, _Die Chaetognathen, eine Monographie_ (Jena, 1880);
B.J. Grassi, _Chetognathi: Flora u. Fauna d. Golfes von Neapel_
(1883); S. Strodtman, _Arch. Naturg._ lviii., 1892; N.M. Stevens,
_Zool. Jahrb. Anat._ xviii., 1903, and xxi., 1905. (A. E. S.)

CHAETOPODA (Gr. [Greek: chaite], hair, [Greek: pous], foot), a
zoological class, including the majority of the Annelida (q.v.), and
indeed, save for the Echiuroidea (q.v.), co-extensive with that group as
usually accepted. They are divisible into the Haplodrili (q.v.) or
Archiannelida, the Polychaeta containing the marine worms, the
Oligochaeta or terrestr