408 PHILOSOPHY OF ZOOLOGY. 



struction of genera and higher divisions. They are inti- 

 mately connected %vith the habits of the animal, and merit 

 the attentive examination of the philosophical naturalist. 



In the molluscous animals the 7iervous system is less 

 complicated in its structure than in the higher classes, and 

 the brain is not restricted in its position to the head. The 

 whole nervous system appears in the form of ganglia and 

 filaments. The principal ganglion, or the one to which the 

 term Brain is usually applied, is seated above the gullet 

 or entrance to the stomach. It sends out nerves to the 

 parts about the mouth, the tentacula, and the eyes. It 

 may be considered as analogous to tlie cerebrum of the 

 vertebral animals. From this ganglion proceed two fila- 

 ments, 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 cerebel- 

 lum, numerous filaments are likewise distributed to the 

 parts around the mouth, and to the other regions of the 

 body. These filaments in some cases again unite, and form 

 subordinate ganglia. In many cases the brain and ganglia 

 fire of a reddish colour, and granulated structure; while 

 the nerves which issue from them are white and uniform, 

 as in the genus Aplysia. The covering of the first gan- 

 ghon, which is analogous to the dura mater, does not ad- 

 here to it closely, but leaves a space filled with loose cellu- 

 lar matter. The tunics of the nerves are equally detached ; 

 and as they can be inflated or injected readily, some have 

 been led to suppose 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 among the 

 mollusca. The touch, that universal sense, is here displayed 

 in many cases with great delicacy; and the tentacula, and the 

 pther cuticular elongations which we have already referre'l 



