70 THE WHENCE AND THE WHITHER OF MAN 



terns is aided by the longer retention of the heat in the 

 body. 



Let us recapitulate. The schematic worm had a 

 body composed of two concentric tubes. The outer 

 was composed of the muscles of the body covered by 

 the protective integument. The inner tube was the 

 alimentary canal with its special muscles. Between 

 these two was the perivisceral cavity, filled with nutri- 

 tive fluid, lymph, and furnishing a safe lodging-place 

 for the more delicate viscera. It represented fairly the 

 trunk of higher animals. 



The annelid added segmentation, and thus greater 

 freedom of motion by the parapodia. But the seg- 

 ments were still practically alike. In the insect divis- 

 ion of labor took place, that is, each group of segments 

 w r as allotted its own special work ; and these groups 

 of segments were modified in structure to best suit the 

 performance of this part of the work of the body. 

 The abdomen was least modified and its eleven seg- 

 ments were devoted to digestion, reproduction, and ex- 

 cretion the old vegetative functions. Three segments 

 were united in the thorax ; all their energy was turned 

 to locomotion, and the insect became thus an exceed- 

 ingly active, swift animal. The third body-region, the 

 head, includes six segments, of which three surrounded 

 the mouth and furnished the jaws, while two more were 

 crowded or drawn forward in order that their ganglia 

 might be added to the old supracesophageal ganglion 

 and form a brain. It is interesting to note that a form, 

 peripatus, still exists which stands almost midway be- 

 tween annelids and insects and has only four segments 

 in the head. The formation of the head was thus a 

 gradual process, one segment being added after another. 



