THE MORPHOLOGY OF THE MOLLUSCA. 13 
these ganglia are placed so close together and are so inti- 
mately connected in the lower Gastropods there is not a 
single case on record in which the pleural ganglion has been 
observed to arise from the pedal ganglion, or from a 
common pleuro-pedal rudiment in the embryo. It is 
equally true on the other hand that Sarasin’s derivation of 
the cerebral and pleural ganglia from a common rudiment 
in Bzthynia (the cephalic sense-plate) has been opposed by 
v. Erlanger, who shows that all the great ganglionic centres 
arise separately, and do not become connected with one 
another until after their differentiation (7). 
A renewed investigation of the origin of the cerebro- 
pleural ganglion in Pelecypoda would be of great interest 
in this connection. Pelseneer’s (18) observations on Vucula 
have placed the fact of the composite nature of this ganglion 
in Pelecypoda beyond all doubt; and still, to the best of 
my knowledge, no one has yet observed the appearance in 
the embryo of a pleural element distinct from the main body 
of the ganglion. This apparent community of origin of the 
cerebral and pleural ganglia in Pelecypoda may be compared 
with the direct continuity of the cerebral and_ pleural 
elements of the nervous system in Amphineura. 
Development of the visceral ganglia. — Sarasin en- 
deavoured to show that the visceral ganglia of Azthynza, 
together with the pedal and abdominal ganglia, arise in the 
embryo from acommon ventral proliferation of the ectoderm 
which he compares with the ventral ganglionic chain of 
Annelida. On this point also Sarasin has been corrected 
by v. Erlanger, who shows that all these ganglia arise 
separately from one another in Lzthynza (7), as well as in 
Paludina (6). | 
The visceral ganglia are also quite distinct from the 
pleural ganglia in their origin, as v. Erlanger’s observations 
show. In one important respect, however, the visceral 
ganglia and the pleural ganglia betray a marked similarity, 
the significance of which seems, however, to have escaped 
