ber of a single pair in most of the segments of the
Chaetopod body, and open each by a ciliated orifice into the coelom on
the one hand, and by a pore on to the exterior of the body on the
other. In its earlier conception, this view embraced as homologous
organs (so far as the present group is concerned) not only the
nephridia of Oligochaeta and Hirudinea, which are obviously closely
similar, but the wide tubes with an intercellular lumen and large
funnels of certain Polychaeta, and (though with less assurance) the
gonad ducts in Oligochaeta and Hirudinea. The function of nitrogenous
excretion was not therefore a necessary part of the view--though it
may be pointed out that there are grounds for believing that the gonad
ducts are to some extent also organs of excretion (see below). Later,
the investigations of E. Meyer and E.S. Goodrich, endorsed by
Lankester, led to the opinion that under the general morphological
conception of "nephridium" were included two distinct sets of organs,
viz. nephridia and coelomoducts. The former (represented by, e.g. the
"segmental organs" of _Lumbricus_) have been asserted to be
"ultimately, though not always, actually traceable to the ectoderm";
the latter (represented by, e.g. the oviduct of _Lumbricus_) are parts
of the coelomic wall itself, which have grown out to the exterior. The
nephridia, in fact, on this view, are _ectodermic ingrowths_, the
coelomoducts _coelomic outgrowths_. The cavity of the former has
nothing to do with coelom. The cavity of the latter is coelom.

The embryological facts upon which this view has been based, however,
have been differently interpreted. According to C.O. Whitman the
entire nephridial system (in the leech _Clepsine_) is formed by the
differentiation of a continuous epiblastic band on each side. The
exact opposite is maintained by R.S. Bergh (for _Lumbricus_ and
_Criodrilus_), whose figures show a derivation of the entire
nephridium from mesoblast, and an absence of any connexion between
successive nephridia by any continuous band, epiblastic or
mesoblastic. A midway position is taken up by Wilson, who asserts the
mesoblastic formation of the funnel, but also asserts the presence of
a continuous band of epiblast from which certainly the terminal
vesicle of the nephridium, and doubtfully the glandular part of the
tube is derived. Vezhdovsky's figures of _Rhynchelmis_ agree with