Studies on Earthworms. - Journal of Cell Science

2 52 WILLIAM BLAXLAND BENHAM.
originally homologous; therefore these glands, whether ossophageal
or intestinal, are homologous. This view cannot, however,
be held if we apply the only true test of homology, that
of common origin from a common ancestor. It is quite clear
that a gland which is in somite xxiv cannot be the same thing
as a gland which existed in somite XIII of an ancestor, or vice
versa. If we are to suppose that similar parts have been
similarly modified for similar wants in different somites, of
two genera of Earthworms compared, then the case is one, not
of homology, but of " homoplasy." (See Lankester, 44.)
The Hervous System.—This seems very similar in all the
worms studied, consisting of a pair of supra-pharyngeal
ganglia, and a series of ventral ganglia, united by cords; besides
these, in at least some forms (Urochseta, Perichseta, Lumbricus,
and others), there is a visceral system of cords and
ganglia on the pharynx, and probably continued farther backwards:
these originate partly from the supra-pharyngeal ganglia
and partly from the circum-pharyngeal commissures.
The presence of the " three great fibres " in the ventral cord
appears pretty constant; but the sub-neural vessel is not
so universal as Claparede supposed; for in Pontodrilus and
Perichseta Houlleti, Perrier has shown that this vessel is
absent [as I shall show later on to be the case in at least one
other worm Microchasta], and as Beddard has shown to be
the case in Pleurochseta. Perrier considers that the supra
pharyngeal ganglia are always in somite m, but he is wrongj
for in Titanus these ganglia lie in somite n, although
dragged back by the muscles of the pharynx; the first ventral
ganglion lies in somite HI, and it is usual to find that ganglion
in the somite following that in which the supra-pharyngeal
ganglia lie.
[In Microchseta and other Earthworms which I shall
describe below, the supra-pharyngeal ganglia lie in somite i
distinctly.] Undoubtedly, most frequently it is the somite in
that in the adult is occupied by the supra-pharyngeal ganglia,
although their seat in embryological origin is the prostomium.
The Vascular System.—In its simplest form, as in Peri-