MOLLUSC A, 



CHAPTER I. 



PHYSIOLOGY. 



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 species ; 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 Tentacula> surround the mouth, and a foot, or in- 

 strument 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 vertebral animals, and containing the brain and organs 

 of the senses, but distinguished merely as the anterior ex- 

 tremity of the body, separated from the back by a slight 

 groove, and containing the mouth and tentacula. 



In many of the animals of this division, the different 



