ial and fresh-water annelids (see EARTHWORM), the
Hirudinea or leeches (see LEECH), and a small group of parasitic worms,
the Myzostomida (q.v.).

The distinctive characters of the class Chaetopoda as a whole are partly
embodied in the name. They possess (save for certain Archiannelida, most
Hirudinea, and other very rare exceptions) setae or chaetae implanted in
epidermal pits. The setae are implanted metamerically in accordance with
the metamerism of the body, which consists of a prostomium followed by a
number of segments. The number of segments in an individual is
frequently more or less definite. The anterior end of body always shows
some "cephalization." The internal organs are largely repeated
metamerically, in correspondence with the external metamerism. Thus the
body cavity is divided into a sequence of chambers by transverse septa;
and even among the Hirudinea, where this condition is usually not to be
observed, there is embryological evidence that the existing state of
affairs is derived from this. Commonly the nephridia are strictly paired
a single pair to each segment, while the branches of the blood vascular
system are similarly metameric. The alimentary canal is nearly always a
straight tube running from the mouth, which is surrounded by the first
segment of the body and overhung by the prostomium, to the anus, which
is then either surrounded by the last segment of the body or opens
dorsally a little way in front of this.

THE CLASS AS A WHOLE.--The Chaetopoda are with but few exceptions
(Myzostomida in part, _Sternaspis_) elongated worms, flattened or, more
usually, cylindrical, and bilaterally symmetrical. The body consists of
a number of exactly similar or closely similar segments, which are never
fused and metamorphosed, as in the Arthropoda, to form specialized
regions of the body. It is, however, always possible to recognize a
head, which consists at least of the peristomial segment with a forward
projection of the same, the prostomium. A thorax also is sometimes to be
distinguished from an abdomen. Where locomotive appendages (the
parapodia of the Polychaeta) exist, they are never jointed, as always in
the Arthropoda; nor are they modified anteriorly to form jaws, as in
that group.

[Illustration: FIG 1.--A, side view of the head region of _Nereis
cultrifera_; B, dorsal view of the same.

E, Eye.
M, Mouth.
d.c, Dorcal cirrus.
per, Peristomium, probably equal to two segments