STRUCTURE OF THE MOUTH. 133 



not very thick, which can join the two edges. This is 

 named a callosity, and thus the complete or terminated 

 shell is constituted. 



The shell of the same individual, however, differs 

 much according to the period of life of the animal 

 which is examined. Generally, the spire is so much 

 the less raised when the animal is younger, and, con- 

 sequently, the last whorl is larger in proportion, the 

 aperture is wider, and the shell is thinner. But when 

 it is terminated, the last whorl comes out from the 

 line of the spiral, and becomes falling, which renders 

 the aperture more narrow. A knowledge of these dif- 

 ferences will prevent our considering individuals of 

 different ages as different species. 



Very variable, according to the food on which they 

 are appointed to subsist, is the structure of the mouth 

 in the gasteropoda. In the snail and its allies the 

 mouth is placed on the under part of the head, and is 

 furnished with an instrument well adapted for cutting 

 the leaves which constitute the animal's food. The oral 

 cavity, which is muscular, has affixed to its upper part a 

 horny plate, the lower margin of which is extremely 

 sharp, cutting like a minute knife, and easily dividing 

 leaves, or biting into soft fruits, when these are worked 

 against it by the action of the lips ; the lower part, 



