VERTEBRATES: BACKBONE AND BRAIN 105 



ments in its en-vironment. Perhaps it is not too mucli 

 to say tliat the eidoscopic eye iirst awakened the slum- 

 bering animal mind, for its reflex effect upon the supra- 

 cesophageal ganglion cannot be OYer-estimated. The 

 animal will very soon begin to think. 



Between the turbellarian and the annelid many aber- 

 rant lines diverged. Some of these attained a compara- 

 tively high level and then seemed to meet insuperable 

 obstacles, while others came to an end or turned down- 

 ward very early. Three of these demanded attention, 

 those leading to mollusks, insects, and vertebrates. 

 And it is interesting to notice that the fundamental 

 difference between these three lines was the skeleton, 

 or perhaps we ought to say it was the habit of life 

 which led to the development of such a skeleton. 



The moUusk took to a sluggish, creeping mode of 

 life, under an external purely protective skeleton ; the 

 insect to a creeping mode of life, with an external but 

 almost purely locomotive skeleton ; the vertebrate 

 kept on swimming and developed an internal locomo- 

 tive skeleton. And it must already have become clear 

 to you that the destiny of these different lines was 

 fixed not so much directly by the skeleton itself as by 

 its reflex effect in moulding the muscular, and ulti- 

 mately the nervous, system. 



The insects formed their skeleton by thickening the 

 homy cuticle of the annelid. They transformed the 

 annelid parapodia into legs and developed wings. 

 They attained life in the air. They devoted the mus- 

 cles of the body largely to the extremities and gained 

 swift locomotion. They have a fair circulatory and an 

 excellent respiratory system. Best of all, they devel- 

 oped a head and a brain by fusing the three anterior 



