MOLLUSCA. 399 



I. 



GANGLIATA 



In this group are included the classes Annulosa and 

 Mollusca. The essential characters, by which these two 

 classes may be distinguished, depend on the condition of 

 the nervous filaments proceeding from the brain, the prin- 

 cipal of which, in the former, constitute a knotted cord pro- 

 ceeding to the extremity, while, in the latter, they separate 

 irregularly. 



Independent of this internal character, molluscous ani- 

 mals are distinguished from those of the annulose division, 

 by the absence of articulated feet, or the cuticular processes 

 which supply their place, and by the body not being di- 

 vided into joints or rings. 



MOLLUSCA. 



Brain surrounding the gullet, and sending out nervous 

 filaments which separate irregularly. 



Molluscous animals exhibit very remarkable differences, 

 both in their form and in the number and position of their 

 external members. Neither head nor foot can be observed in 

 some, the principal organs being enclosed in a bag pierced 

 with apertures for the entrance of the food, and egress of 

 the excrementitious matter. In others, with an exterior 

 still remarkably simple, cuticular elongations, termed Ten- 

 tacula, surround the mouth, and a foot, or instrument of 

 motion, may likewise be perceived. This last organ is in 

 some free at one extremity, in others attached to the body 

 throughout its whole length. In many species there is a 

 head, not, however, analogous to that member in the ver- 

 tebral animals, and containing the brain and organs of the 

 senses, but distinguished merely as the anterior extremity of 



