30 DIGESTIVE ORGANS. 



little teeth spreading out. The tongue rubs off particles of 

 nourishment only in the process of retraction, or tears larger 

 pieces from leaves for example, and draws them into the oral 

 cavity. In seizing and holding the nourishment, the strong 

 annular lip and the jaws are useful accessories. 



As already stated, certain corresponding peculiarities of ani- 

 mals and shells enable us to separate the carnivorous from the 

 phytophagous mollusks ; curiously enough, the arrangement of 

 the lingual ribbon does not indicate this separation, and we ac- 

 cordingly find, in systems of classification based upon this organ, 

 the animal and vegetable feeders rather incongruously mixed. 



Besides the mastication of food, the teeth are probably used 

 in boring through the shells of other mollusks in order to obtain 

 the flesh. It is still a matter of discussion whether this opera- 

 tion is effected by mechanical or chemical action or by a combi- 

 nation of both ; but it is generally supposed that the teeth are 

 the tools by which an excavation through the hard shell of the 

 victim is perforated. Most of the large siphonostomate Proso- 

 branchiates obtain food in this manner, as well as the Naticas 

 among the holostomates ; and I shall have occasion frequently 

 to refer to the subject hereafter when treating of the individual 

 species. The shells attacked are usually bivalves, which are 

 bored near the beaks where they are thinnest. That instinct is 

 sometimes at fault in these creatures is evidenced by the solid 

 spine of a sea urchin, which P. P. Carpenter relates, has been 

 bored through by a mollusk. 



On every coast the evidence of this work of destruction is 

 abundant, a large portion of the bivalves washed ashore, being 

 perforated near the umbones. 



Miiller has seen Cerithia on the Brazilian coast bored by 

 Murex Senegalensis, in consequence of which the animal dies 

 and opens its operculum, when a Turbinella comes to share the 

 feast. He has seen a dozen specimens of Cerithium at one time 

 with the Murex extracting the meat through a boring in the 

 spire and the Turbinella at work within the aperture of the shell. 

 When both these are done, a Pagurus occupies the empty shell, 

 or shares it with a Crepidula.* 



* Jena Zeit., 57, 1871. 



