MOLLUSCA. 3 
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 
