ved not only by the correspondence in number to
them of the gonads, but by actual observation of the generative
products in transit. This form of nephridia leads to the shorter but
essentially similar organs in the Polychaete _Sternaspis_, and to
those of the Echiuroidea (q.v.) and of the Gephyrea (q.v.).

Though the paired arrangement of the nephridia is the prevalent one in
the Chaetopoda, there are many examples, among the Oligochaeta, of
species and genera in which there are several, even many, nephridia in
each segment of the body, which may or may not be connected among
themselves, but have in any case separate orifices on to the exterior.

2. _Coelomoducis._--In this category are included (by Goodrich and
Lankester) the gonad ducts of the Oligochaeta, certain funnels without
any aperture to the exterior that have been detected in _Nereis_, &c.,
funnels with wide and short ducts attached to nephridia in other
Polychaeta, gonad ducts in the _Capitellidae_, the gonad ducts of the
leeches. In all these cases we have a duct which has a usually wide,
always intercellular, lumen, generally, if not always, ciliated, which
opens directly into the coelom on the one hand and on to the exterior
of the body on the other. These characters are plain in all the cases
cited, excepting only the leeches which will be considered separately.

There is not a great deal of difference between most of these
structures and true nephridia. It is not clear, for example, to which
category it is necessary to refer the excretory organs of _Arenicola_,
or _Polynoe_. Both series of organs consist essentially of a ciliated
tube leading from the coelom to the exterior. Both series of organs
grow back centrifugally from the funnel. In both the cavity originally
or immediately continuous with the coelom appears first in the funnel
and grows backwards. In some cases, e.g. oviducts of Oligochaeta,
sperm ducts of _Phreoryctes_, the coelomoducts occupy, like the
nephridia, two segments, the funnel opening into that in front of the
segment which carries the external pore. It is by no means certain
that a hard and fast line can be drawn between intra- and
intercellular lumina. Finally, in function there are some points of
likeness. The gonad ducts of _Lumbricus_, &c., must perform one
function of nephridia; they must convey to the exterior some of the
coelomic fluid with its disintegra