HOW ANIMALS EAT. 



65 



movement is given 

 to the horny ridges, 

 so that the "bite" 

 of the Leech is real- 

 ly a saw-cut. 



The dentition of 



B 



Fro. 2y. Teeth and Masticatory Apparatus of Gastero- 

 Univalve Mol- pods: A, portion of odontophore, or " tongue," of FeJ- 

 utina, enlarged ; B, portion of odontophore of Whelk 

 (Bwxinum undatnm), magnified the .entire tongue 

 has 100 rows of teeth ; C. head and odontophore of Lim- 

 pet (Patella vulgata) ; D, portion of same, greatly mag- 

 nified, to show the transverse rows of siliceous teeth. 



lusks, or the Snails, 

 is generally lingual, 

 *. e. 9 it consists of 

 microscopic teeth, 



usually siliceous and amber -colored, 

 planted in rows on the tongue. 

 The teeth are, in fact, the ser- 

 rated edges of minute plates. 

 The number of these plates va- 

 ries greatly; the garden Slug 

 has 160 rows, with 180 teeth 

 in each row. 



All living Birds, and some 

 other Vertebrates, as Ant-eat- 

 ers, 24 Turtles, Tortoises, Toads, 



1 and Sturgeons, have no teeth. 



Their place is often supplied 

 by a horny beak, a muscular 

 gizzard, or both structures. 



In a few Vertebrates, horny 

 plates take the place of teeth, 

 as the Duck Mole (Ornitho- 

 rhynchus) and Whalebone 

 Whale. In the former, the 

 plates consist of closely set ver- 

 hollow tubes; in the lat- 

 he baleen, or whalebone, 

 attaching the horny body of the~ba- plates, triangular in shape, and 



leen-plate,c; d, fringe of bristles; e, * . 



emaiier plates. fringed on the inner side, hang 



5 



tfio. 30. Section of one half of the Tip- 

 per Jaw of a Whale (Balcenoptera), 

 showing baleen - plates : a, superior er 

 maxillary bone; 6, ligamentous gum ' 



