s is thought but
not known, this double origin, is termed a nephromixium. The various
facts, however, seem to be susceptible of another interpretation. It
may be pointed out that the several examples described recall a
phenomenon which is not uncommon and is well known to anatomists. That
is the replacement of an organ by, sometimes coupled with its partial
conversion into, a similar or slightly different organ performing the
same or an analogous function. Thus the postcaval vein of the higher
vertebrata is partly a new structure altogether, and is partly formed
out of the pre-existing posterior cardinals. The more complete
replacements, such as the nephridia of the genital segment of
_Tubifex_ by a subsequently formed genital duct, may be compared with
the succession of the nesonephros to the pronephros in vertebrates,
and of the metanephros to the mesonephros in the higher vertebrates.
It might be well to term these structures, mostly serving as gonad
ducts, which have an undoubted resemblance to nephridia, and for the
most part an undoubted connexion with nephridia, "Nephrodinia," to
distinguish them from another category of "ducts" which are
communications between the coelom and the exterior, and which have no
relation whatever to nephridia or to the organs just discussed. For
these latter, the term coelomoducts might well be reserved. To this
category belong certain sacs and pouches in many, perhaps most, genera
of the Oligochaeta family, _Eudrilidae_, and possibly the gonad ducts
in the Hirudinea. As an example of the former it has been shown
(Beddard) that a large median sac in _Lybiodrilus_ is at first freely
open to the coelom, that it later becomes shut off from the same, that
it then acquires an external orifice, and, finally, that it encloses
the ovary or ovaries, between which and the exterior a passage is thus
effected. To this category will belong the oviducts in Teleostean
fishes and probably the gonad ducts in several groups of
invertebrates.

POLYCHAETA.--This group may be thus defined and the definition
contrasted and compared with those of the other divisions of the
Chaetopoda. Setae always present and often very large, much varied in
form and very numerous, borne by the dorsal and ventral parapodia (when
present). The prostomium and the segments generally often bear processes
sensory and branchial. Eyes often present and comparatively compl