re is a single pair of large anterior
nephridia, which open by a common pore, followed after an interval by
large-funnelled and short nephridia. This differentiation is not,
however, peculiar to the Polychaetes; for in several Oligochaetes the
anterior nephridia are of large size, and opening as they do into the
buccal cavity clearly play a different function to those which follow.
In _Thamnodrilus_, as has been pointed out, there are two series of
nephridia which resemble those of the Terebelloidea in the different
sizes of their funnels. In _Lanice conchilega_ the posterior series of
nephridia are connected by a thick longitudinal duct, which seems to
be seen in its most reduced form in _Owenia_, where a duct on each
side runs in the epidermis, being in parts a groove, and receives one
short tubular nephridium only and occupies only one segment. This
connexion of successive nephridia (in _Lanice_) has its counterpart in
_Allolobophora, Lybiodrilus_, and apparently in the Lumbriculids
_Teleuscolex_ and _Styloscolex_, among the Oligochaeta. Among the
_Capitellidae_, which in several respects resemble the Oligochaeta,
wide and short gonad ducts coexist in the same segments with
nephridia, the latter being narrower and longer. It is noteworthy that
in this family only among the Polychaeta, the nephridia are not
restricted to a single pair in each segment; so that the older view
that the gonad ducts are metamorphosed nephridia is not at variance
with the anatomical facts which have been just stated.

_Alimentary Canal._--The alimentary canal of Polychaetes is usually a
straight tube running from the anterior mouth to the posterior anus.
But in some forms, e.g. _Sternaspis_, the gut is coiled. In others,
again, e.g. _Cobangia_, the anus is anterior and ventral. A gizzard is
present in a few forms. The buccal cavity is sometimes armed with
jaws. The oesophagus is provided often with caeca which in Syllids and
_Hesionidae_ have been found to contain air, and possibly therefore
perform the function of the fish's air-bladder. In other Polychaetes
one or more pairs of similar outgrowths are glandular. The intestine
is provided with numerous branched caeca in _Aphrodite_.

[Illustration: FIG. 4.--_Dasychone infracta_, Kr. (After Malmgren.)]

_Reproduction._--As is the case with the Oligochaeta, the Polychaeta
furnish examples of species which multiply asexuall