e. They contrast with those of the Oligochaeta and
Hirudinea by reason of their frequently close association with the
gonads, the same organ sometimes serving the two functions of
excretion and conveyance of the ova and spermatozoa out of the body.
On the hypothesis that such a form as _Dinophilus_ (see Haplodrili)
has preserved the characters of the primitive Chaetopod more nearly
than any existing Polychaet or Oligochaet, it is clear that the
nephridia in the Oligochaeta have preserved the original features of
those organs more nearly than most Polychaeta. Thus _Nereis_ among the
latter worms, from the resemblance which its excretory system bears to
that of the Oligochaeta, may be made the starting-point of a series.
In this worm the paired nephridia exist in most of the segments of the
body, and their form (see fig. 2) is much like that of the nephridia
in the _Enchytraeidae_. The funnel, which is not large, appears to
open, as a rule at least, into the segment in front of that which
bears the external orifice. Quite independent of these are certain
large dorsally situate funnel-like folds of the coelomic epithelium,
ciliated, but of which no duct has been discovered leading to the
exterior. It is possible that we have here gonad ducts distinct from
nephridia which at the time of sexual maturity do open on to the
exterior.

In _Polynoe_ the nephridia are short tubes with a slightly folded
funnel whose lumen is intercellular, and this intercellular lumen is
characteristic of the Polychaetes as contrasted with leeches and
Oligochaetes. Among the Terebelloidea there is a remarkable
differentiation of the nephridia into two series. One set lies in
front of the diaphragm, which is the most anterior and complete
septum, the rest having disappeared or being much less developed. The
anterior nephridia, of which there are one to three pairs, contrast
with the posterior series by their small funnels and large size, the
posterior nephridia having a large funnel followed by a short tube. In
_Chaetozone setosa_ the anterior nephridia occupy five segments. There
is usually a gap between the two series, several segments being
without nephridia. It seems that the posterior nephridia are mainly
gonad ducts, and the gonads are developed in close association with
the funnels. The same arrangement is found in some other Polychaetes;
for instance, in _Sabellaria_ the