64 
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 
inent of levers and muscles. Instead of moving up and 
down, as in Yertebrates, or from right to left, as in Ar¬ 
thropods, they converge towards the centre, and the food 
passes between ten grinding surfaces. 
The Kotifers (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 
