104 



NUTRITION. 



amidst this diversity of apparatus, there is one thing which 

 characterizes all the Articulata, namely, the jaws always 

 move sideways ; while those of the Vertebrates and Mollusks 

 move up and down, and those of the Radiata concentrically. 



215. In the Vertebrates, the iaws form a part of the bony 



skeleton. In most of them the 

 lower jaw only is movable, and 

 is brought up against the upper 

 jaw by means of very strong mus- 

 cles, the temporal and masseter 

 Fig. 67. muscles, (Fig. 67, t, m,} which 



perform the principal motions requisite for seizing and mas- 

 ticating food. 



216. The jaws are usually armed with solid cutting 



instruments, the TEETH, or else are 

 enveloped in a horny covering, the 

 beak, as in the birds and tortoises, 

 (Fig. 68.) In some of the whales, 

 the true teeth remain concealed in the 

 Fig. 68. jaw-bone, and we have instead a range 



of long,* flexible, horny plates or fans, fringed at the margin, 

 which serve as strainers to separate the minute marine ani- 

 mals on which they feed 

 from the water drawn in 

 with them, (Fig. 69.) 

 A few are entirely des- 

 titute of teeth, as the 

 ant-eater, (Fig. 70.) 



217. Though all the 

 vertebrates possess jaws, 

 Fig. 69. it must not be inferred 



that they all chew their food. Many swallow their prey 

 whole ; as most birds, tortoises, and whales. Even many of 

 those which are furnished with teeth do not masticate their 



