mbricus_ (in its widest sense) by
Vezhdovsky. It is further noticeable that in _Rhynchelmis_ the
covering of vesicular cells which clothes the drain-pipe cells of the
adult nephridium is cut off from the nephridial cells themselves and
is not a peritoneal layer surrounding the nephridium. Thus the
nephridia, in this case at least, are a part of the coelom and are not
shut off from it by a layer of peritoneum, as are other organs which
lie in it, e.g. the gut. A growth both of the funnel, which becomes
multicellular, and of the rest of the nephridium produces the adult
nephridia of the genera mentioned. The paired disposition of these
organs is the prevalent one among the Oligochaeta, and occurs in all
of twelve out of the thirteen families into which the group is
divided.

Among the _Megascolicidae_, however, which in number of genera and
species nearly equals the remaining families taken together, another
form of the excretory system occurs. In the genera _Pheretima,
Megascolex_, _Dichogaster_, &c., each segment contains a large number
of nephridia, which, on account of the fact that they are necessarily
smaller than the paired nephridia of e.g. _Lumbricus_, have been
termed micronephridia, as opposed to meganephridia; there is, however,
no essential difference in structure, though micronephridia are not
uncommonly (e.g. _Megascolides_, _Octochaetus_) unprovided with
funnels. It is disputed whether these micronephridia are or are not
connected together in each segment and from segment to segment. In any
case they have been shown in three genera to develop by the growth and
splitting into a series of original paired pronephridia. A complex
network, however, does occur in _Lybiodrilus_ and certain other
_Eudrilidae_, where the paired nephridia possess ducts leading to the
exterior which ramify and anastomose on the thickness of the body
wall. The network is, however, of the duct of the nephridium, possibly
ectodermic in origin, and does not affect the glandular tubes which
remain undivided and with one coelomic funnel each.

The Oligochaeta are the only Chaetopods in which undoubted nephridia
may possess a relationship with the alimentary canal. Thus, in
_Octochaetus multiporus_ a large nephridium opens anteriorly into the
buccal cavity, and numerous nephridia in the same worm evacuate their
contents into the rectum. The anteriorly-opening and usual