180 



BIOLOGY FOR BEGINNERS 



made to bend by the same arrangements, only better adapted than 

 man's, and they cover their head and body by a shield (carapace) 

 far lighter and more efficient than ever warrior carried. Not 

 only is their exo-skeleton strong, light, and flexible, but it is colored 

 so as to escape notice from enemies (protective coloration). It 

 is also provided with sharp spines and projects downward at the 

 sides, thereby guarding the gills and soft under parts of the body. 

 In addition to its protective function, the skeleton forms a rigid 



. FIG. 57. The crayfish a type of Crustacea. 



series of levers, by means of which a complicated system of muscles 

 provides for swift motion and locomotion, essential for escape, 

 attack, and food-getting. The development of a skeleton has 

 also enabled its possessor to advance in many ways. 



As stated before, the body consists of segments (20 in all) though 

 only the abdominal ones are movable, those of the cephalo- thorax 

 being fused (united) together for greater strength and protection, 

 while the numerous appendages provide for the needed freedom of 

 motion. Each of these twenty segments has a pair of appendages, 



