362 LIFE PROCESSES OF ANIMALS 



which are useful in carrying on the work of the body. 

 These glands are the organs of secretion antl the chief ones 

 are connected with the (hgestive tract and ijerform very 

 important functions in the process of tligestion. For ex- 

 ample, the salivary glands opening into the mouth, where 

 the saliva dissolves and moistens the food and, in some 

 animals, acts upon the starch, changing it into sugar. In 

 the walls of the stomachs of the higher niollusks and the 

 invertebrates there are glands called gastric joUides that 

 secrete the gastric juice. In the Protozoa there are no 

 special glands for secreting digestive fluids and the whole 

 work qf digestion is accomplished by the protoplasm. The 

 same may be said of the sponges and hydrse except that 

 in the latter certain cells of the endoderm secrete a diges- 

 tive fluid. The liver and pancreas are two very important 

 organs of secretion connected with the alimentary canal 

 of many animals. The liver of vertebrates is the largest 

 gland in the body. It secretes bile, the work of which is 

 imperfectly understood, Init it certainly performs an im- 

 portant fvuiction in digestion. 



Excretion. — Thei'e are two antagonistic processes going 

 on in the Ijodies of all animals during life; namely, a buikl- 

 ing-up process and a tearing-tlown process.. In the tearing- 

 down process, waste materials are given off into the fluids of 

 the body — l)lood in those animals that have blood. These 

 waste materials, which consist mainly of carbonic acitl, 

 water, and m-ea, must be thrown off from the body, or 

 excreted. In the vei-tel>rates, they are thi'own off by 

 the lungs, skin, and kidneys. In the lo\A'er animals, we do 

 not find all of these special oi'gans of excretion. Foi' 

 example, the amodia excretes carbonic acid through the 

 sui'faceof the botly and the pulsating vacuole aids in cxcret- 



