those of Bergh in showing the backward growth of the nephridium from
the funnel cell. There are thus substantial reasons for believing that
the nephridium grows backwards from a funnel as does the coelomoduct.
It is therefore by no means certain that so profound a difference
embryologically can be asserted to exist between the excretory
nephridia and the ducts leading from the coelom to the exterior, which
are usually associated with the extrusion of the genital products
among the Chaetopoda.

There are, however, anatomical and histological differences to be seen
at any rate at the extremes between the undoubted nephridia of
Goodrich, Meyer and Lankester, and the coelomoducts of the same
authors.

[Illustration: FIG. 2. (from Goodrich).

A, Diagram of the nephridium of _Nereis diversicolor_.
B, Diagram of the nephridium of _Alciope_, into which opens the large
genital funnel (coelomostome).
C, Small portion of the nephridium of _Glycera siphonostoma_, showing
the canal cut through, and the solenocytes on the outer surface.
D, Optical section of a branch of the nephridium of _Nephthys
scolopendroides_.
c.s, Cut surface.
cst, Coelomostome.
f, Flagellum.
g.f, Genital funnel.
n, Neck of solenocyte.
n.c, Nephridial canal.
n.p, Nephridiopore.
nst, Nephridiostome.
nu, Nucleus of solenocyte.
s, Solenocytes.
t, Tube.]

I. _Nephridia_.--Excretory organs which are undisputed nephridia are
practically universal among the Oligochaeta, Hirudinea and
Archiannelida, and occur in many Polychaeta. Their total absence has
been asserted definitely only in _Paranais littoralis_. Usually these
organs are present to the number of a single pair per somite, and are
commonly present in the majority of the segments of the body, failing
often among the Oligochaeta in a varying number of the anterior
segments. They are considerably reduced in number in certain
Polychaeta. Essentially, a nephridium is a tube, generally very long
and much folded upon itself, composed of a string of cells placed end
to end in which the continuous lumen is excavated. Such cells are
termed "drain pipe" cells. Frequently the lumen is branched and may
form a complicated anastomosing network in these cells. Externally,
the nephridium opens by a straight part of the tube, which is often
very wide, and here the intracellular lumen