stages m Hitman Evolution 



became harder, the struggle more severe, com- 

 petition more marked and harsh. A strong, 

 tough, agile, alert body was to be developed. 

 Worms led the way toward this. But they had 

 only begun to utilize and realize the possibili- 

 ties of the muscular system. As soon as this and 

 the visceral organs needed for its support and 

 service had been fairly started, the worm be- 

 gan to experiment in building a skeleton. Three 

 forms of skeleton are possible. 



First comes the external skeleton used by mol- 

 lusks and other animals as a means of protec- 

 tion. The shells of snails and clams are good 

 illustrations. Second, there Is the external loco- 

 motive skeleton, a framework for the attach- 

 ment of muscles which move the jointed levers 

 of the legs. This is well illustrated by the horny 

 covering of insects. It is really, while somewhat 

 protective, a passive organ of locomotion. In 

 the lobster and many crabs it becomes so heavy 

 as to hinder locomotion and to be largely pro- 

 tective. The third form is the internal skeleton 

 used originally almost entirely for locomotion. 

 This is illustrated by our backbone. Ribs and 

 cranium are protective structures of later origin. 

 Many vertebrates, turtles and armadillos, have 



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