OF DIGESTION. 



77 



the food to chyle which render their digestive apparatus 

 quite complicated. In the first place, hard parts, of a horny 

 or bony texture, are usually placed ahout the mouth of 

 those animals that feed on solid substances, which serve 

 for cutting or bruising the food into small fragments before 

 it is swallowed ; and, in many of the lower animals, these 

 organs are the only hard portions of the body. This pro- 

 cess of subdividing or chewing the food, is termed masti- 

 cation. 



212. Beginning with the Radiata, we find the apparatus 

 for mastication partaking of the star-like arrangement which 



Fig. 55. Fig. 56. 



characterizes those animals. Thus, in Scutella (Fig. 55), 

 we have a pentagon composed of five triangular jaws, con- 

 verging at their summits towards a central aperture which 

 corresponds to the mouth, each one bearing a plate or tooth, 

 like a knife-blade, fitted by one edge into a cleft. The five 

 jaws move towards the centre, and pierce or cut the objects 

 which come between them. In some of the sea-urchins, 

 (Echinus), this apparatus, which has been called Aristotle's 

 lantern (Fig. 56), consists of numerous pieces, and is 

 much more complicated. Still, the five fundamental pieces 

 or jaws, each of them bearing a tooth at its point, may be 

 recognized as in Scutella ; only instead of being placed hori- 

 zontally, they form an inverted pyramid. 



7* 



