188 



OEGA5s'S OF DIGESTION. 



199) ; or it is stiff, and folded beneath the chest, as in the 

 squash-bugs (fig. 197), containing several piercers of extreme 

 delicacy (fig. 198), adapted to penetrate the skin of animals or 

 other objects whose juices they extract; or the parts of the mouth 

 are prolonged, so asto shield the tongue when thrust out insearch 

 of food, as in the bees (fig. 196, j, p). The crabs have their 



Fig. 195. Fig. 196. Fig, 197. Fig. 198. Fig. 199. 



anterior feet transformed into jaws, and several other pairs of 

 articulated appendages perform exclusively masticatory func- 

 tions. Even in the microscopic rotifera, we find very com- 

 plicated jaws, as seen in the interior of Esophora (fig. 172), 

 But amidst this diversity of apparatus, there is one circum- 

 stance which characterizes all the articulata, namely, the 

 jaws move sideways ; while those of the vertebrata and mol- 

 lusca move up and down, and those of the radiata concen- 

 trically. 



338. In the vertebrata, the jaws form a part of the 

 bony skeleton. In most of them the lower jaw (fig. 103) 

 only is moveable, and is brought up against the upper jaw by 

 means of the temporal and masseter muscles, which perform 

 the principal motions requisite for seizing and masticating 

 food. 



339. The jaws are usually armed with solid cutting in- 

 -p. . struments, the TEETH, or else are enveloped 



in a horny covering, the beak, as in birds 

 and tortoises (fig. 200). In some of the 

 whales, the true teeth remain concealed in 

 the jaw bone, and they have instead, a range 

 of long, flexible, horny plates or fans, fringed 

 at the margin, serving as strainers to separate 

 the minute marine animals on which they feed from the water 

 drawn in with them (fig. 201). A few are entirely destitute of 

 teeth, as the ant-eaters (fig. 202). 



