s to two pairs of
setae or even to a single seta. On the other hand, in certain
Polychaeta the bundles of setae are so extensive that they nearly form
a complete circle surrounding the body; and in the Oligochaet genus
_Perichaeta_ (= _Pheretima_), and some allies, there is actually a
complete circle of setae in each segment broken only by minute gaps,
one dorsal, the other ventral.

_Coelom_.--The Chaetopoda are characterized by a spacious coelom,
which is divided into a series of chambers in accordance with the
general metamerism of the body. This is the typical arrangement, which
is exhibited in the majority of the Polychaeta and Oligochaeta; in
these the successive chambers of the coelom are separated by the
intersegmental septa, sheets of muscle fibres extending from the body
wall to the gut and thus forming partitions across the body. The
successive cavities are not, however, completely closed from each
other; there is some communication between adjoining segments, and the
septa are sometimes deficient here and there. Thus in the Chaetopoda
the perivisceral cavity is coelomic; in this respect the group
contrasts with the Arthropoda and Molluscs, where the perivisceral
cavity is, mainly at least, part of the vascular or haemal system, and
agrees with the Vertebrata. The coelom is lined throughout by cells,
which upon the intestine become large and loaded with excretory
granules, and are known as chloragogen cells. Several forms of cells
float freely in the fluid of the coelom. In another sense also the
coelom is not a closed cavity, for it communicates in several ways
with the external medium. Thus, among the Oligochaeta there are often
a series of dorsal pores, or a single head pore, present also among
the Polychaeta (in _Ammochares_). In these and other Chaetopods the
coelom is also put into indirect relations with the outside world by
the nephridia and by the gonad ducts. In these features, and in the
fact that the gonads are local proliferations of the coelomic
epithelium, which have undergone no further changes in the simpler
forms, the coelom of this group shows in a particularly clear fashion
the general characters of the coelom in the higher Metazoa. It has
been indeed largely upon the conditions characterizing the Chaetopoda
that the conception of the coelom in the Coelomocoela has been based.

Among the simpler Chaetopoda the coelom