MOLLUSCA. 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 
of 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 szght 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 
