36 COMPARATIVE DENTAL ANATOMY 



other, so as to insure continued sharpness from 

 wear. They are set in alveoli of bony structure, 

 and are moved by sets of strong muscles in vari- 

 ous directions. The entire apparatus consists of 

 twenty pieces, — i.e., five teeth, five alveoli, five 

 rotalse, and five radii. It is concealed within the 

 test in life with only the points of the teeth pro- 

 jecting, which are very effective for cutting shells, 

 boring into rocks, and reducing food substances. 



The Annuloida comprise the segmented worms, 

 some of which possess so-called teeth, but these 

 partake more of the nature of serrated jaws than 

 of teeth. These jaws are located on the second 

 or buccal segment, which may be protruded from 

 the mouth a considerable distance. These jaws 

 are of chitinous structure, commonly paired and 

 of an infinite variety of forms. In the Leeches, 

 the mouth is provided with three lenticular jaws, 

 with the projecting edges finely serrated. The 

 medicinal leech has two rows of serrations, which 

 make three radiating slits. The strong suctorial 

 power draws an eminence of skin into the mouth, 

 which is slit by the serrated jaws. 



The Nereis and Philodace have strong jaws like 

 the carnivorous beetles, which are cruelly effective 

 in attacking lower Invertebrates (Fig. 3). Their 

 jaws are serrated and opposite, and are worked 

 by powerful muscles. 



In the Arthropoda — including the Insects and 



