ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA l8l 



ganglia. Considering, however, that the auditory nerves are invariably 

 attached to the pedal ganglia in all other mollusks, and that in Ptero- 

 ceras and Strombiis, genera which so nearly approach the Heteropoda, 

 the auditory nerves are very long, I do not think it very hazardous to 

 suppose that in the Heteropoda the auditory nerves really proceed 

 from the pedal ganglia, but have become united to the cephalic 

 ganglia. 



In any other case their position is quite exceptional, for the supra- 

 oesophageal position of the auditory sacs in Nudibranchiata merely 

 arises from the pedal ganglia being thrust upwards, and united with 

 the cephalic ganglia. 



The accessory ganglia of the pedal ganglion appear to be only 

 what may be called digital ganglia, developed to meet the wants of 

 certain elongations or expansions of the foot. 



Such are the ganglia at the bases of the arms of the Cephalopoda, 

 and such appear to me to be the ganglia which supply the " labial " 

 processes of Nautilus. 



III. Under the name of Parieto-splanchnic system of ganglia, I in- 

 clude the branchial and visceral ganglia of most authors, and the 

 cervical, branchio-cardiac, and angeial ganglia of M. Blanchard. This 

 system consists of two primary ganglia, which are always to be found 

 at the side of the oesophagus, connected with both the pedal and 

 cephalic ganglia, and for which I reserve specially the term parieto- 

 splanchnic ganglia ; from these nerves are given off — 



1. Parietal, to the sides of the body, as distinct from the foot. 



2. Columellar, to the shell-muscle or muscles, of which there are 

 two in Octopus, Nautilus, and Cymbulia ; one in the shelled Gastero- 

 poda. 



3. Branchial, to the branchiae. 



4. Angeial, to the heart and great vessels and generative organs. 



Separate ganglia, answering to the three latter sets of nerves, may 

 be found in the dibranchiate Cephalopoda ; to the two last in the 

 Heteropoda ; a single ganglion corresponding to all of them is found 

 in Aplysia, Buccinum, Turbo, Paludina, &c. Two such exist in 

 Strombus and Pteroceras. 



The angeial ganglia, wherever they exist separately, are placed 

 above the aorta and united by a commissure. 



The visceral or sympathetic nerves ramify extensively over the 

 intestinal canal (see Hancock and Embleton upon Doris). They are 

 connected anteriorly with the buccal ganglia, posteriorly with the 

 parieto-splanchnic system. 



