238 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
nature, but also supplies a point of analogy between the nervous system of the Mol- 
lusca and that of the Vertebrata, which was previously wanting. We are now free 
to compare the nervous centres and their offsets grouped round the oesophagus in 
the Mollusca, with those which constitute the cerebro-spinal system of the Vertebrata. 
This we shall now proceed to do ; and it is highly interesting to find how remarkably 
close the analogy is, even to the details, between the sets of organs in the two Sub- 
kingdoms. But first let us take a brief review of the ganglia and their relation to 
each other in Doris, and then of the relation of the ganglia in Doris to those in other 
Mollusca. It has been believed by Cuvier and others that these ganglia are to a 
greater or less degree fused into one mass ; but if in D. tubercidata the contents of 
the capsule* or dura mater be pressed out completely, the true manner of the con- 
nection becomes evident. It is then seen that the communication between the ganglia 
is kept up through narrow apertures by means of small oval commissures, and that 
the masses are closely pressed as it were together, the opposed surfaces being flat. 
In such a preparation it can be observed that the cerebroid ganglia are divided by a 
strong septum, which is perforated by an oval opening ; that the branchial are con- 
nected to both the cerebroid and the pedial ganglia by commissures somewhat 
similar ; that the pedial are similarly united to the cerebroid ; and lastly, that the 
visceral ganglion is joined to the branchial through a small aperture. The third or 
principal collar round the oesophagus is in such a preparation distinctly seen to be 
divided into three strands, two of which enter the pedial ganglia, the other passes 
from the left branchial to the visceral, which again, as above stated, is connected 
with the right branchial. From these intercommunications between the ganglia, and 
from the distributions of the nerves arising from them, a strict analogy may be 
drawn between the nervous centres of Doris, in which they have attained a high 
degree of concentration, and those of other Moilusks in which they are comparatively 
disjoined. We may take Aplysia kyhrida as a fair specimen of the latter class. Here 
the cerebroid ganglia are slightly apart, being united by a short commissure. These, 
as in Doris, give their branches to the dorsal tentacles, the eyes, the channel of the 
mouth and lips. At a little distance behind these centres are the branchial, which 
are on the same plane, considerable in volume, rounded, and connected with the 
former by a stout commissure. The branchial ganglia supply the mantle, and are 
doubtless the horaologues of the parts of the same name in Doris. Backwards from 
them pass two stout trunks, which go to two ganglia in the vicinity of the branchiie ; 
these ganglia are little inferior in size to the branchial, are united to each other by a 
distinct commissure, and send their nerves to the generative organs, heart and intes- 
tine ; they are in some species fused into one, as we see in Cuvier’s memoirs, and 
this he believed to have the office of a sympathetic system. These ganglia, or this 
ganglion, from its connections with the branchial, and from the distribution of its 
nerves, is undoubtedly the homologue of our single visceral ganglion in Doris. The 
* Plate XVI. fig. 9. 
