266 



COMPARATIVE ZOOLOGY 



other by a complex arrangement of levers and muscles. 

 Instead of moving up and down, as in vertebrates, or 

 from right to left, as in arthropods, they converge 

 toward the center, and the food passes between ten 

 grinding surfaces. 



The rotifers have a curious pair of horny jaws. That 

 which answers to the lower jaw is fixed, and called the 



" anvil." The upper jaw 

 consists of two pieces called 

 "hammers," which are 

 sharply notched, and beat 

 upon the " anvil " between 

 them (Fig. 40). 



The horny-toothed mandi- 

 bles 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 saw- 

 ing movement is given to 

 the horny ridges, so that 

 the " bite " of the leech is 

 really a saw cut. 



The dentition of the uni- 

 valve mollusks, or the snails, 

 is generally lingual, i.-e., it 

 consists of microscopic teeth, usually siliceous and 

 amber-colored, planted in rows on the tongue. The 

 teeth are, in fact, the serrated edges of minute plates. 



FIG. 228. Section of one half of the Up- 

 per Jaw of a Whale (Balasnoptera), 

 showing baleen plates: a, superior 

 maxillary bone; 6, ligamentous gum 

 attaching the horny body of the baleen 

 plate, c; d, fringe of bristles ; e, smaller 

 plates. 



