Nervous System and Senses. 527 



those of the common fresh-water muscle are invariably bright 

 yellow.* 



The brain, in the cephalous Mollusca is composed of two 

 similar parts, more or less closely united by a sort of com- 

 missure. In the Cephalopoda, it is enclosed in a cavity in 

 the posterior part of the cartilage of the head, which is 

 pierced by a number of holes to give passage to the nerves ; 

 but, in the Gasteropoda and others, it lies, protected only by 

 the skin, above the oesophagus, and behind an oval mass of 

 muscles which envelopes the mouth and pharynx. From the 

 brain, the nerves which go to the tentacula and lips originate ; 

 it sends anastomosing branches to the ganglia of the other 

 organs of sense, when they exist ; and from it likewise pro- 

 ceeds a remarkable filament which surrounds the gullet like 

 a collar. 



The nerves of the skin, and of the organs of locomotion, 

 issue from a single ganglion situated on each side, sometimes 

 placed at a considerable distance from the brain, with which 

 it always communicates by a nervous cord ; more often so 

 near that it seems a part of the same organ. In both cases, 

 it is from this ganglion only that all the nerves depart which 

 are distributed over the musculo-cutaneous envelope, and 

 particularly to those parts of it which are subservient to loco- 

 motion, as to the foot of the Gasteropoda, the sac of the cut- 

 tlefish tribe, the fins of the Pteropoda, &c. 



The visceral ganglia appear to be only two in number : one, 

 supplying nerves to the organ peculiar to the male, is situated 

 near the orifice by which this issues ; the other, more con- 

 stantly present, is usually placed near the stomach, and fur- 

 nishes the intestinal canal with its nerves. Both ganglia send 

 also branches of communication to the brain or oesophageal 

 collar. 



It may seem unnecessary to mention that the developement 

 of the diflPerent parts of the nervous system is in proportion 

 to that of the organs to which they belong ; that, consequently, 

 it is much more perfect in the cuttlefish, which are at the 

 head of the class, than in the Patellae, which are at the bot- 

 tom of it. The observation applies equally to the nervous 

 system of the acephalous Mollusca, in which, indeed, it is 

 so little developed, that the very existence of it was for a long 

 time not perceived. In them, the brain is a double ganglion, 

 or rather a sort of flattened cord, situated always above the 

 oesophagus, and from which the nerves in general originate. 

 There does not appear to be any proper oesophageal collar; but 



* Comp. Anat., t ans., i. 53. 



