GENERAL ORGANOLOGT. 115 



variable and animals with definite temperatures. Under the head 

 of animals with varying temperature (poikilothermous) or cold 

 blood are placed forms whose temperature is largely dependent 

 upon the temperature of the environment, rising and falling with 

 it, but usually a few degrees above it. In our climate, where the 

 atmospheric temperature is considerably lower than the tempera- 

 ture of the human body, such animals, for example the frog, 

 would feel cold to our touch, since they, particularly in the cool 

 season, have a much lower temperature than we. 



Such creatures as, living under any thermal condition, maintain 

 about the same temperature, are termed warm-blooded or definite- 

 temperatured (idiothermous, homoiothermous) animals. Man in 

 summer and winter, under the equator and at the north pole, has 

 approximately a temperature of 36 C. (98| F.), showing higher 

 temperatures only in fever. In order to maintain a constant tem- 

 perature during the varying external conditions, the animal must 

 have a heat-regulator; it must have the power to regulate the 

 warmth of its body, on the one hand by limiting the production of . 

 heat, on the other by controlling its loss. If the environment be 

 warmer than is suitable for the body temperature, then the pro- 

 duction of heat must be limited to the smallest quantity com- 

 patible 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 active perspiration. If, on the contrary, 

 the environment be 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 requires compli- 

 cated apparatus, can occur only in the highly organized animals. 



IV. Excretory Organs. 



Nature of the Organs of Excretion. The excretory organs are 

 tubes or glandular canals which open upon the surface of the body, 

 either directly or by way of an end-gut (cloaca), and conduct sub- 

 stances which have become useless to the body to the exterior. 



The presence of a blood-vascular system or a coelom or both 

 together exercises an important influence on their structure. 

 When neither are developed the excretory tubules in order to 

 remove the excreta from the tissues must branch and penetrate 

 the body in all directions like a drainage system, being frequently 

 connected in a network recalling the blood-capillaries (proto- 

 nephridia or water-vascular st/stem of parenchymatous worms, 



