252 WILLIAM BLAXLAND BENHAM. 
originally homologous; therefore these glands, whether ceso- 
phageal or intestinal, are homologous. This view cannot, how- 
ever, 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 xxrv cannot be the same thing 
as a gland which existed in somite x111 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 Nervous 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 (Urocheta, Pericheta, Lum- 
bricus, and others), there is a visceral system of cords and 
ganglia on the pharynx, and probably continued farther back- 
wards: 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 Claparéde supposed; for in Pontodrilus and 
Pericheta 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 Microcheta], and as Beddard has shown to be 
the case in Pleurocheta. Perrier considers that the supra 
pharyngeal ganglia are always in somite 111, but he is wrong, 
for in Titanus these ganglia lie in somite 1, although 
dragged back by the muscles of the pharynx; the first ventral 
ganglion lies in somite 111, and it is usual to find that ganglion 
in the somite foliowing that in which the supra-pharyngeal 
ganglia lie. 
[In Microcheta and other Earthworms which I shall 
describe below, the supra-pharyngeal ganglia lie in somite 1 
distinctly.] Undoubtedly, most frequently it is the somite m1 
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- 
