48 THE WHENCE AND THE WHITHER OF MAN 



The animal can gain sufficient oxygen to supply its 

 muscles and nerves, which are the principal seats of 

 combustion, through the external surface. It has, 



therefore, no special respir- 

 atory organs. But the waste 

 matter of the muscles cannot 

 escape so easily, for these 

 are becoming deeper seated. 

 Hence we find an excretory 

 system consisting of two 

 tubes with many branches 

 in the parenchyin, and dis- 

 charging at the rear end of 

 the body. This again is a 

 sign that the muscles are 

 becoming more important, 

 for the excretory system is 

 needed mainly to remove 

 their waste. These tubes 

 may be only greatly enlarged 

 glands of the skin. 



The nervous system con- 

 sists of a plexus of fibres 

 and cells, the cells originat- 

 ing impulses and the fibres 

 5. TURBELLARIAN. LANG. conveying them. But this 



va and ha, front and rear branches mil c}l was present in hydra 

 of gastro - vascular cavity ; ph, L 



pharynx. The dark oval with also. Here tlie Il'Ollt end 



fine branches represents the ner- Q fa Q body gO6S foremost 

 vous system. , 



and is continually coming in 



contact with new conditions. Here the lookout for 

 food and danger must be kept. Hence, as a result of 

 constant exercise, or selection, or both, the nerve- 



