136 VARIETIES OF STRUCTURE. 



each other by the action of muscles. The tongue is armed 

 with hard spines, rendering it a pointed file ; and when 

 it is applied by the proboscis to any shell the animal 

 desires to drill, the supporting cartilages by their move- 

 ments alternately depress and elevate these spines, which 

 rasp away, on a small surface, and soon penetrate 

 through the substance. Perhaps, in this operation the 

 saliva, which is carried by long ducts to the tongue, 

 assists, by some solvent quality it may possess ; but 

 though this is very probable, it is not positively ascer- 

 tained : certain it is, that with this slender rasp-like 

 tongue, worked by its supporters, the whelk will pierce 

 shells of great solidity. 



Such are the principal conditions of the structure 

 of the mouth in the gasteropoda ; but the modifications 

 of each pattern are nearly as numerous as the species 

 themselves ; in all, however, the mouth is a positive and 

 direct agent in obtaining food. A very marked difi'er- 

 ence may here be seen between these creatures and the 

 bivalve mollusks, in which a syphon, and the cilia of 

 the branchise propel a stream of water, laden with nu- 

 triment, to the mouth of an inert being ; the mouth 

 opening for the reception of such particles as are fitted 

 for the animal's digestion. 



Another remarkable fact must not be overlooked. 



