488 Mr. Garner on the Nervous System of Molluscous Animals. 



In Pecten, Spondylus and Ostrea we find small, brilliant, emerald-like 

 ocelli, which, from their structure, having each a minute nerve, a pupil, a pig- 

 nientum, a striated body, and a lens, and from their situation at the edge of 

 the mantle, where alone such organs could be useful, and also placed, as in 

 Gasteropoda, with the tentacles, must be organs of vision. 



The Gasteropoda offering much variety in form, present likewise corre- 

 sponding differences in the nervous system ; for in all animals the disposition 

 of the latter is chiefly determined by the shape of the body ; keeping it, how- 

 ever, in mind, that as we ascend, we find an inclination in the several ganglia 

 to become concentrated and ascend towards the head. With the exception of 

 the Tunicata, we find in all Mollusca the centre of the nervous system to be a 

 ring around the commencement of the digestive tube, more narrowly em- 

 bracing it as we get higher in the orders of animals. Its exact situation 

 varies with circumstances ; thus it will be found around the very commence- 

 ment of the alimentary canal, close to the lips, in Helix ; in Eollda behind the 

 muscular pharynx ; some distance down the oesophagus in Buccimtm ; whilst 

 in one species of Purpura it is generally behind the stomach. In these latter 

 animals it is fixed itself, but the oesophagus has free motion through it, as the 

 proboscis is more or less protruded. 



The nervous system of Patella (Tab. XXV. fig. 3.) shows, on the one hand, 

 the resemblance of this system in the Gasteropoda to that of the Conchlfera ; 

 and on the other to that of the Cephalopoda. We have the cerebral or sentient 

 ganglia (A, A.) at the base of the tentacles and eyes, which are here present, 

 supplying principally those organs, and receiving a filament (b.) on each side 

 from the pedal ganglion (B.), and another (a.) from the branchial ganglion (C), 

 as in Conchifera. The ganglia of both these jmirs are connected that their 

 action may be combined, the connecting filament of the branchial passing in 

 its course through the two pedal ganglia. The pedal ganglion supplies the 

 foot, the branchial or respiratory ganglion the branchiae and mantle, also 

 giving lesser filaments (/.) to the viscera, and others (e.) to the shell muscles. 

 That this last is a branchial ganglion is proved by this : the author has ob- 

 served that in Flssurella (an animal differing from Patella in having the 

 branchiae removed to the back of the neck, and in which animal Cuvier 

 notices the deficiency of the two external ganglia,) they exist, but in a dif- 



