176 MASTICATING INSTRUMENTS OF INVERTEBRATA. 



ever, the tooth seems like a continuation of the bone of the 

 jaw, not being in any way separated from it, and the tubular 

 structure of the latter being continued into it without any 

 interruption. The teeth of fishes are often set, not only upon 

 the proper jaw-bones, but upon the surface of the palate, and 

 even in the pharynx or swallow. 



189. In the Invertebrata there are generally no proper 

 teeth ; in the Articulated and sometimes in the Molluscous 

 series, however, we meet with firm horny jaws, which are 

 often furnished with projections that answer the same pur- 

 pose ; and in most Gasteropods we find a very curious organ, 

 commonly designated as the tongue, more correctly the 

 palate, the surface of which is beset with innumerable tooth- 

 like points (fig. 106), by whose rasping action the food is 

 reduced. These teeth present great varieties of form and 



arrangement in the different 

 genera and species of this group ; 

 and these varieties appear to 

 bear some relation to the nature 

 of the food on which the animals 

 respectively live. It is remark- 

 able that in an animal so low 

 in the scale as the Echinus or 

 Sea- Urchin ( 1 19), a very com- 

 plex dental apparatus should 

 exist. This consists of five long 

 hard teeth, which surround the 

 mouth ; and these are fixed in 

 a framework which is worked 



- 



and thus serve effectually to grind down the food. 



Insalivation. 



190. The act of mastication is connected with another; 

 which is also of great importance in preparing for the sub- 

 sequent process of digestion. This is the blending of the 

 saliva with the food, during its reduction between the teeth, 

 an act which is termed insalivation. The saliva is separated 

 from the blood, by glands which are situated in the neigh- 



