icated
in structure. Clitellum not present as a definite organ, as in
Oligochaeta. The anus is mostly terminal, and there are no anterior and
posterior suckers. Nervous system often imbedded in the epidermis.
Vascular system generally present forming a closed system of tubes.
Alimentary canal rarely coiled, occasionally with glands which are
simple caeca and sometimes serve as air reservoirs; jaws often present
and an eversible pharynx. Nephridia sometimes of the type of those of
the Oligochaeta; in other cases short, wide tubes with a large funnel
serving also entirely or in part as gonad ducts. Frequently reduced in
number of pairs; rarely (_Capitellidae_) more than one pair per segment.
Gonads not so restricted in position as in Oligochaets, and often more
abundant; the individuals usually unisexual. No specialized system of
spermathecae, sperm reservoirs, and copulatory apparatus, as in
Oligochaeta; development generally through a larval form; reproduction
by budding also occurs. Marine (rarely fresh-water) in habit.

The Polychaeta contrast with the Oligochaeta by the great variety of
outward form and by the frequency of specialization of different regions
of the body. The head is always recognizable and much more conspicuous
than in other Chaetopoda. As in the Oligochaeta the peristomial segment
is often without setae, but this character is not by any means so
constant as in the Oligochaeta. The prostomium bears often processes,
both dorsal and ventral, which in the Sabellids are split into the
circle of branchial plumes, which surround or nearly surround the mouth
in those tube-dwelling Annelids. _Tomopteris_ is remarkable for the fact
that the hammer-shaped prostomium has paired ventral processes each with
a single seta. It is held, however, that these are a pair of parapodia
which have shifted forwards. The presence of parapodia distinguish this
from other groups of Chaetopoda. Typically, the parapodium consists of
two processes of the body on each side, each of which bears a bundle of
setae; these two divisions of the "limb" are termed respectively
notopodium and neuropodium. The notopodium may be rudimentary or absent
and the entire parapodium reduced to the merest ridge or even completely
unrepresented. Naturally, it is among the free living forms that the
parapodium is best developed, and least developed among the tubicolous
Polychaeta. To each division of the parapodium belongs typically a long
tentacle, t