COMPARATIVE ZOOLOGY. 



purpose, as in the Horse -shoe Crab; or the stomach is 

 lined with "gastric teeth," as in some marine Snails; or 

 the deficiency is supplied by a muscular gizzard, as in 

 Birds, Ant-eaters, and some Insects. Even the Lobster 

 and Crab, in addition to their complicated oral organs, 

 have the stomach furnished with a powerful set of teeth. 

 The Sea-urchin is the first of animals, and almost 

 the only one below Worms and Mollusks, which exhibits 



anything like a 

 dental apparatus. 

 Five calcareous 

 teeth, having a 

 wedge - shaped 

 apex, each set in 

 a triangular pyr- 

 amid, or "jaw," 

 are moved upon 

 each other by a 

 complex arrange- 

 ment of levers and muscles. Instead of moving up and 

 down, as in Vertebrates, or from right to left, as in Ar- 

 thropods, they converge towards the centre, and the food 

 passes between ten grinding surfaces. 



The Rotifers (a group of minute Worms) have a curi- 

 ous pair of horny jaws. That which answers to the lower 

 jaw is fixed, and called the " anvil." The upper jaw con- 

 sists of two pieces called " hammers," which are sharply 

 notched, and beat upon the " anvil " between them (Fig. 

 219). 



The horny-toothed mandibles of Insects, already men- 

 tioned, are prehensile, and also serve to divide the food. 



The three little white ridges in the mouth of the Leech 

 are the convex edges of horny semicircles, each bordered 

 by a row of nearly a hundred hard, sharp teeth. When 

 the mouth, or sucker, is applied to the skin, a sawing 



FIG. 28. Sea-urchin bisected, showing masticating appara- 

 tus. 



