MOIXUSCA. 13 



animals. From this ganglion proceed two filaments, one 

 on each side, which in their descent inclose the gullet, and 

 unite underneath to form a second ganglion. From this 

 last, which has been compared to the cerebellum, numerous 

 filaments are likewise distributed to the parts around the 

 mouth, and to the other regions of the body. These fila- 

 ments in some cases again unite, and form subordinate gan- 

 glia. In many cases the brain and ganglia are of a reddish 

 colour, and granulated structure, while the nerves which 

 issue from them are white and uniform, as in the genus Ap- 

 lysia. The covering of the first ganglion, which is analo- 

 gous to the dura mater, does not adhere to it closely, but 

 leaves a space filled with loose cellular matter. The tu- 

 nics of the nerves are equally detached ; and as they can 

 be inflated or injected readily, some have been led to sup- 

 pose that the nerves were hollow, and others, that the tunics 

 were the vessels of the lymphatic system. 



The organs of perception common to the higher classes 

 x>f animals, do not all exist in an obvious manner amongst 

 the mollusca. The touch, that universal sense, is here dis- 

 played in many cases with great delicacy ; and the tenta- 

 cula, and the other cuticular elongations which we have al- 

 ready referred to, contribute to augment its resources. The 

 sense of sight is by no means universally enjoyed by the in- 

 habitants of this class. In a few species, the eye is con- 

 structed on the plan of the same organ in the vertebral ani- 

 mals. In general, however, it appears only as a black point, 

 the peculiar functions of which can only be inferred from 

 analogy. In many species there is no trace of an eye, con- 

 sequently they cannot possess that varied information which 

 the others derive from that organ. Where eyes exist in 

 this class, they are uniformly two in number. In one tribe 



