MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
239 
remaining supra-oesophageal ganglia are the pedial. These are placed on a plane 
below the branchial, on the side of the oesophagus, and are connected with the 
cerebroid and branchial by stout commissures of considerable length. Three large 
nerves are given to the foot from these ganglia. Thus we find that the homology 
of the parts is complete; and were the ganglia concentrated, we should find them 
connected in precisely the same manner as in Doris. There is moreover an exact 
correspondence as to the infra-oesophageal ganglia and their nerves in Doris and 
Aplysia. 
The examination of the nervous centres of Arion and Onchidium strongly support 
the view we take of the functions of the different nervous ganglia in Doris and 
Aplysia. In both these instances we have the cerebroid ganglia giving their nerves 
to the organs of the senses, and to the channel of the mouth and lips, the branchial 
giving off their nerves to the mantle, and the pedial supplying the foot. Arion and 
Onchidium, however, differ from each other as regards the visceral ganglion ; but 
this difference itself only corroborates our views of the functions of that organ. In 
Arion it is still formed, as in Aplysia, of two large centres ; but these, instead of being 
placed at a distance and connected by long commissures with the branchial, are 
placed close between and united to them, and overlie the pedial. In Onchidium 
there is fusion of the pair into a single ganglion, which lies in the same position with 
regard to both branchial and pedial, and has the same connection with the branchial 
as in Arion. The nerves arising from this centre in Onchidium, and from this pair 
of centres in Arion, we have traced, though not quite perfectly, to all the viscera as 
in Doris. In Onchidium a little alteration in the relative position of the ganglia is 
all that is necessary to make the resemblance complete between its nervous centres 
and those of Doris. 
Taking this view of things, we find ourselves at issue to some extent with M. E. 
Blanchard ; for instance, the large posterior nerve from the branchial ganglion, the 
cervico-cardiac of this author, sends a branch of communication, as we have seen, to 
unite with the branchial sympathetic ganglia ; these ganglia he takes to be the same 
as those in the posterior part of the body of Aplysia, and which we believe to be the 
homologues of our visceral ganglion; hence it is that he was led to consider the 
connectives between these ganglia and the supra-oesophageal centres to be the homo- 
logues of the great posterior nerve from the branchial ganglion ; but in Aplysia we 
have the true representative of this nerve, which arises as in Doris from the branchial 
ganglion, and supplies the posterior part of the mantle. Moreover, we have dis- 
covered near the roots of the respiratory organ in Aplysia a minute ganglion and 
nerves, the real counterparts of the sympathetic branchial ganglion of Doris. We 
have therefore little or no doubt of the accuracy of our views on this point. 
If we compare the nervous system of the higher Mollusks with that of the Lamelli- 
branchiata, we shall find that the principal centres are sufficiently well represented 
in the lower forms. In Mya truncata we have two large ganglia placed anteriorly at 
2 I 
MDCCCLII. 
