MOUTHS OF MOLLUSCA. 



but it often forms, in its own substance, calca- 

 reous laminae, or plates of various degrees of density, 

 and which are deposited in layers, as in the mantle 



Mantle of Dolium galea, with the spiral part of the animal in outline, 

 to show the position. 



of the slug ; but the term is still retained, as this 

 deposit is not visible externally. When, however, the 

 mantle secretes an external shell, forming a defensive 

 habitation, of which we shall give hereafter many in- 

 stances, the animals are termed testaceous, or shelled 

 moUusks. This division, however, is one of mere con- 

 venience, and not scientific. 



The mouths of each species of mollusca, like all their 

 other organs, are fully adapted to their respective cir- 

 cumstances. Some need only a little opening, to admit 

 the animalcules borne to them by the water ; and this, 

 therefore, we observe. Those which subsist on vege- 

 table productions have mouths provided with jaws, which 



