SECRETION AND EXCRETION. 



121 



active the animal and its breathing, the higher its temper- 

 ature. Birds and Mammals have a constant temperature, 

 which is usually higher than that of the atmosphere (108° 

 and 100° F. respectively). They are therefore called con- 

 stant-temperatured or warm-blooded. Other animals do 

 not vary greatly in temperature from that of their sur- 

 roundings, and are called changeable-temjperatured or cold- 

 blooded. Still, their temperature does not agree exactly 

 with that of the air or water. The Bee is from 3° to 10°, 

 and the Earth-worm and Snail from 1^° to 2°, higher than 

 the air. The mean temperature of the Carp and Toad is 

 51°; of Man, 98°; Dog, 99°; Cat, 101°; Squirrel, 105°; 

 Swallow, 111°. 



CHAPTER XV. 



SECRETION AND EXCRETION. 



In the circulation of the blood, not only are the nutrient 

 materials deposited within the body in the form of tissue, 

 but certain special fluids are separated and conveyed to 

 the external or internal surfaces of the body. These flu- 

 ids are of two kinds : some, like saliva, gastric juice, bile, 

 milk, etc., are for useful purposes ; others, like sweat and 

 urine, are expelled from the system as useless or injurious. 

 The separation of the former is called secretion; the re- 

 moval of the latter is excretion. Both processes are sub- 

 stantially alike. 



In the lower forms, there are no special organs, but se- 

 cretion and excretion take place from the general surface. 

 The simplest form of a secreting organ closely resembles 

 that of a respiratory organ, a thin membrane separating 

 the blood from the cavity into which the secretion is to 



