274 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



The higher position of insects is further shown by the 

 number of their legs or appendages used for locomotion. 

 It is a law in biology that a great number of parts, simi- 



FiG. 175.' — Lithobius forcipatus. (After Carpenter.) 



lar in form and function (vegetative repetition), indicates 

 a low position in the scale of organisms. As we go up 

 the scale the number of parts used for one function be- 

 comes less, and their efficiency becomes correspondingly 

 greater. Legs are an admirable illustration of this law. 

 In marine worms and in the lowest Crustacea there is an 

 indefinite but very great number of similar legs. As we 

 rise among Crustacea the number becomes definite, and 

 countable as legs, when there are fourteen or seven pairs. 

 These are called tetradecapods. In the higher Crusta- 

 cea — crabs, crawfish, etc. — they are reduced to five pairs. 



Fig. 176. — Syllis prolifera. 



These are therefore called decapods. In spiders and scor- 

 pions there are only four pairs. These might be called 

 octopod insects. In true insects they are reduced to three 



