176 MASTICATING INSTRUMENTS OF INVERTEBRATA. 
ever, the tooth seems like a continuation of the hone 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 
hear 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 (§ 119), 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 
by a powerful set of muscles, 
and thus serve effectually to grind down the food. 
Fig.106. —Dental Organ of Nerita. 
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- 
