138 



GENERAL PRINCIPLES OF ZOOLOGY. 



regulate the warmth of its body, on the one hand by lim- 

 iting the production of heat, on the other by controlling 

 the loss of heat. If the environment is warmer than is 

 suitable for the body temperature, then the production of 

 heat must be limited to the smallest quantity compatible 

 with the vital processes; but, if this does not suffice, the 

 loss of heat must be increased by evaporation from the 

 surface, usually accomplished by lively perspiration. If, 

 on the contrary, the environment is cold, then, conversely, 



every unnecessary loss of heat must 

 be avoided, while the production of 

 heat must be increased. It is clear 

 that idiothermy, since it necessitates 

 complicated apparatus, can occur 

 only in the highly organized animals. 



IV. Excretory Organs. 



Nature of the Organs of Ex- 

 cretion. The excretory organs are 

 tubes or glandular canals which open 

 upon the surface of the body, either 

 directly or by the mediation of 



ft 



FIG. 64. FlG - 6s " 



FIG. 6i,.Distoma hefaticum with water-vascular system. /, porus excretorius ; <?, mouth- 

 opening-. (From Hatschek ) 



FIG 65 Blind end of one of the finest water-vascular canals (k\ of a Turbellanan, (trorn 

 Lang.) , nucleus; f, projection of the terminal cell ; wf, "flame" of the terminal 

 cell ; v, vacuole. 



an end-gut (cloaca), and conduct to the exterior substances 

 which have become useless to the body. The presence 



