han is apparent in some
Chaetopods. The external segments are usually definable by the setae;
and if the setae are absent, as in the anterior segments of several
_Geoscolicidae_, the nephridiopores indicate the segments; to each
segment corresponds internally a chamber of the coelom which is
separated from adjacent segments by transverse septa, which are only
unrecognizable in the genus _Aeolosoma_ and in the head region of
other Oligochaeta. In the latter case, the numerous bands of muscle
attaching the pharynx to the parietes have obliterated the regular
partition by means of septa.

_Nephridia_.--The nephridia in this group are invariably coiled tubes
with an intracellular lumen and nearly invariably open into the coelom
by a funnel. There are no renal organs with a wide intercellular
lumen, such as occur in the Polychaeta, nor is there ever any
permanent association between nephridia and ducts connected with the
evacuation of the generative products, such as occur in _Alciope_,
_Saccocirrus_, &c. In these points the Oligochaeta agree with the
Hirudinea. They also agree in the general structure of the nephridia.
It has been ascertained that the nephridia of Oligochaeta are preceded
in the embryo by a pair of delicate and sinuous tubes, also found in
the Hirudinea and Polychaeta, which are larval excretory organs. It is
not quite certain whether these are to be regarded as the remnant of
an earlier excretory system, replaced among the Oligochaeta by the
subsequently developed paired structures, or whether these "head
kidneys" are the first pair of nephridia precociously developed. The
former view has been extensively held, and it is supported by the fact
that in _Octochaetus_ the first segment of the body has a pair of
nephridia which is exactly like those which follow, and, like them,
persists. On the other hand, in most Oligochaeta the first segment has
in the adult no nephridium, and in the case of _Octochaetus_ the
existence of a "head kidney" antedating the subsequently developed
nephridia of the first and other segments has neither been seen nor
proved to be absent. In any case the nephridia which occupy the
segments of the body generally are first of all represented by paired
structures, the "pronephridia," in which the funnel is composed of but
one cell, which is flagellate. This stage has at any rate been
observed in _Rhynchelmis_ and _Lu