OF DIGESTION. 



79 



and contains several piercers of extreme delicacy, (Fig. 63), 

 adapted to penetrate the skin of animals or other objects 

 whose juices they extract ; or they are prolonged so as to 

 shield the tongue when thrust out in search of nutritive 

 juices, as in the bees (Fig. 61.) The crabs have their 

 anterior feet transformed into a kind of jaws. Indeed, 

 even down to the microscopic Rotifers, we find veiy 

 complicated jaws, as seen in the interior of Brachionus 



Fig. 65. 



Fig. 66. 



(Fig. 65), and represented largely magnified in Fig. 66. 

 But amidst this diversity of apparatus, there is one thing 

 which characterizes all the Articulata, namely, the jaws all 

 move sideways ; while those of the Vertebrates move up and 

 down, and those of the Radiata move concentrically. 



215. In the Vertebrates, the jaws 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, , m), which per- 



form all the motions requisite for seizing and masticating food. 



216. The jaws are usually armed 

 with solid cutting instruments, the 

 TEETH, or else enveloped in a horny 

 covering, the leak, as in the birds and 

 tortoises (Fig. 68). In some of the 



whales, we have instead, a range of Fig. 68. 



