lOG THE VITAL FUNCTIONS. 



likewise materially contribute; although their pri- 

 mary and more usual office is the mechanical 

 division of the food by means of mastication, an 

 action in which the jaws, in their turn, co-operate. 

 Another principal purpose effected by the jaws is 

 that of giving mechanical power to the muscles, 

 which, by acting upon the sides of the cavity of the 

 mouth, tend to compress and propel the contained 

 food. We find, accordingly, that all animals of 

 a highly developed structure are provided with 

 jaws.* 



Among the animals which are ranked in the 

 class of Zoophytes, the highest degrees of develope- 

 ment are exhibited by the Echinodermata ; and in 

 them we find a remarkable perfection in the organs 

 of mastication. The oesophagus of the Echinus is 

 surrounded by a framework of shell, consisting of 

 five converging pieces, each armed with a long 

 tooth ; and for the movement of these parts there 

 are provided twenty separate muscles, of which the 

 anatomy has been minutely described by Cuvier. 

 In the shells of the echini which are cast upon the 

 shore, this calcareous frame is usually found entire 

 in the inside of the outer case; and Aristotle having 

 noticed its resemblance to a lantern, it has often 

 gone by the whimsical name of the lantern of Aris- 

 totle. 



In all articulated animals which subsist on solid 

 aliment, the apparatus for the prehension and 

 mastication of the food, situated in the mouth, is 

 exceedingly complicated, and admits of great di- 



* Ehrenberg has found tliat a regular apparatus of jaws, armed 

 with rows of teeth, exists in most of the rotatory infusoria, and even 

 in some species of polygastrica. Ann. Sc. Nat. serie 2, iii. 281. 



