240 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
the sides of the oesophagus, connected by a longish commissure ; these, which are 
usually called the labial, answer to the cerebroid of Doris, and give branches to the 
anterior margins of the mantle, the seats of sensation ; and the corresponding ganglia 
in some species are said to have the auditory capsules attached to them. From these 
centres run backward two commissures, one of which runs into a ganglion placed in the 
muscles of the foot, the pedial, the other passing further back enters a large ganglion 
situated on the posterior adductor muscle. This mass sends large nerves to the 
branchiae, the siphonal tubes and the posterior portion of the mantle. This is the 
branchial ganglion of authors, and as the ganglia which we have so called in Doris, 
supply nerves to the mantle, and are brought into connection with the branchial 
ganglia of the sympathetic system ; we believe them to be the homologues of the last- 
named ganglion of the Lamellibranchiata. It is worthy of note that we have disco- 
vered in Mya truncata two small elliptical ganglia attached to the anterior and 
underpart of the branchial, and united together by a commissure. These send fila- 
ments to the ovary, and ventricle of the heart, and therefore probably represent the 
visceral ganglia of Doris. Thus we have the representation of the nervous centres of 
Doris complete in Mya. 
We shall now, then, return to the comparison of the nervous oesophageal ganglia in 
Doris with the ganglia making up the cerebro-spinal system of the Vertebrata. 
First, then, we have a small anterior pair of ganglia, which we have called olfactory, 
for reasons given in our anatomy of Eolis, and which it is perhaps unnecessary to 
repeat here. This is in position, and probably in function, the counterpart of the 
same organ in the Vertebrata, the rhinencephalon of Professor Owen. Secondly, we 
have the large anterior supra-oesophageal or sensorial or cerebroid ganglia supplying 
the lips and channel of the mouth with both motor and sensitive nerves, and not only 
supplying the olfactory, but also the optic ganglia and the auditory capsules. This 
second pair is clearly comparable to either the hemispherical ganglia or the centres 
of sensation and volition, or to both these sets together, with a portion of the anterior 
extremity of the spinal cord, and probably also the cerebellum. They are analogous 
therefore to the prosencephalon, a portion of the meseneephalon and of the anterior 
end of the rnyelon, and possibly also of the epencephalon of the same high authority. 
Thirdly, we have the optic ganglia ; these come the next in order in the Mollusca as 
in the Vertebrata, and in the former are to all appearance dependences of the senso- 
rial or cerebroid. The optic ganglia from which the optic nerves arise, are analogous 
to the corpora quadrigemina, or a portion of the mesencephalon. Fourthly, come the 
branchial centres ; these are next the median line, on the same plane as the cere- 
broid and posterior to them, and in several species, as we have seen, more or less 
joined with them into one mass. The nerves from these ganglia go to the respiratory 
organs, that is the skin or mantle, and to the ganglionic nervous centres of the 
branehiee. The branchial ganglia correspond in function to the pneumonic portion 
of the pneumogastric apparatus of the medulla oblongata or macromyelon of the 
