310 



MOVEMENT IN LOWER ANIMALS 



arranged in pairs, as in our own bodies. In the crus- 

 taceans these legs may be as many as twenty, some being 

 moved backward as the others 

 move forward. The motion of the 

 six legs of the insect is illustrated 

 in Figure 142. 



Most land vertebrates, except 

 the snakes, have four legs, the two 

 front ones corresponding to our 

 arms, and the two hind ones to 

 our legs. In some forms the legs 

 are so placed that the animal 

 stands upon the tips of the toes 

 as in the horse, cat, dog, etc. In others, as in man and 

 the bears, the whole foot is on the ground. Nearly all 



FiO. 142 — Motion of the legs 

 of a fly in crawling. 



Fig. 143 — Feet of mammals adapted to different metliods of locomotion ; A, foot 

 of bear ; B, foot of seal ; C, foot of lion. 



vertebrates have more powerful leg muscles than man, 

 and can move much more rapidly. The legless snakes 



