GLANDS, SENSE ORGANS 151 



350. The epidermis of Crustacea and Insects is generally 

 devoid of glands. Fishes and Amphibia are well supplied with 

 slime glands. Reptiles have practically no glands in the skin. 

 In Birds there is one important gland or group of glands. This 

 is the uropygal gland, located on the tail. It secretes an oil 

 which is transferred by the bird by means of its beak to the 

 surface of the feathers when preening. The oil keeps the 

 feathers flexible and prevents wetting. For this reason 

 " water rolls off a duck's back/' as it also does from other 

 birds. 



351. The skin of Mammals is provided with two kinds of 

 glands, sebaceous and sweat glands. The former are grouped 

 around the hair follicles and their oily secretion escapes at the 

 base of the hair. It not only serves to keep the hair flexible 

 but also the corneous layer of the epidermis. This is a very 

 important function, since the dead tissue would otb : ~e 

 break and expose the living tissue beneath, which at 

 happens in the chapping of an abnormally "dry" skiii v*ie 

 oil also renders the skin impervious to water. 



352. The sweat glands are the thermostatic organs of the 

 body. They will be discussed elsewhere. (Page 411.) 



SENSE ORGANS 



353. Aru'mals which live in the water may sense their food 

 in one or more of three ways, viz. : Sight, smell, and taste. For 

 amoeba the first is excluded, since amoeba cannot be said to 

 have a sense of sight. The sensations of taste and smell are 

 both due to chemical stimuli; that is to say, the substances 

 which stimulate the sense organs must be in solution, and the 

 stimulation itself is a chemical process. The distinction be- 

 tween the two sets of senses is largely a matter of position of the 

 sense organs. The organs of taste are located in the mouth, while 

 those of smell are elsewhere on the surface of the body, usually 



