he cirrus, which may be defective upon one or other of the
notopodium or neuropodium, and may be developed into an arborescent gill
or into a flat scale-like process, the elytron (in _Polynoe_, &c.).
There are other gills developed in addition to those which represent the
cirri.

_Setae_.--The setae of the Polychaeta are disposed in two bundles in
many genera, but in only one bundle in such forms as have no
notopodium (e.g. _Syllis_). In some genera the setae are in vertical
rows, and in certain _Capitellidae_ these rows so nearly meet that an
arrangement occurs reminiscent of the continuous circle of setae in
the perichaetous Oligochaeta. The setae vary much in form and are
often longer and stronger than in the Oligochaetes. Jointed setae and
very short hooks or "uncini" (see fig. 3) are among the most
remarkable forms. Simple bifid setae, such as those of Oligochaetes,
are also present in certain forms.

[Illustration: Fig. 3.--a, Bristle of _Pionosyllis Malmgreni_; b, Hook
of _Terebella_.]

Among the burrowing and tubicolous forms it is not uncommon for the
body to be distinguishable into two or more regions; a "thorax," for
example, is sharply marked off from an "abdomen" in the Sabellids. In
these forms the bundles of setae are either capilliform or uncinate,
and the dorsal setae of the thorax are like the ventral setae of the
abdomen. It is a remarkable and newly-ascertained fact that in
regeneration (in _Potamilla_) the thorax is not replaced by the growth
of uninjured thoracic segments; but that the anterior segments of the
abdomen take on the same characters, the setae dropping out and being
replaced in accordance with the plan of the setae in the thorax of
uninjured worms. Among the Oligochaeta the sexually mature worm is
distinguished from the immature worm by the clitellum and by the
development of genital setae. Among the Polychaeta the sexual worm is
often more marked from the asexual form, so much so that these latter
have been placed in different species or even genera. The alteration
in form does not only affect structures used in generation; but the
form of the parapodia, &c., alter. There are even dimorphic forms
among the Syllids where the sexes are, as in many Polychaets,
separate.

_Nephridia_.--The nephridia of the Polychaeta have been generally
dealt with above in considering the nephridial system of the
Chaetopoda as a whol