CHAPTER XIX 



THE EXCRETORY OR NEPHRIDIAL TISSUES 



THE life of all animals depends upon a double process by which com- 

 plex tissue substances are built up to be broken down later in the release 

 of some kind of energy. This double process of building up and breaking 

 down is known as metabolism. It involves the securing and distribution 

 of food and the collection and elimination of waste products resulting 

 from the breaking down of tissue materials. Tissues of alimentation 

 and circulation take care of the securing and distributing of the food. 

 The nitrogenous waste products of metabolism are poisonous to the 

 tissues not differentiated for their reception, and must be readily removed 

 from the animal body or be stored 

 in some tissue highly modified for 

 their reception. Tissues of urine 

 excretion, therefore, have evolved 

 along with the advance in ani- 

 mal structure. In all cases, ex- 

 cept in the ascidians, these 

 tissues are organized in such a 

 manner that the nitrogenous 

 waste products can get out from 

 the body. These structures vary 

 much in their complexity. 



Among the unicellular forms 

 we have seen that the alimenta- 

 tion and distribution of food 

 was effected by means of vacu- 



Oles Within the Cytoplasm Of the FIG. 301. Two infusorians swimming in a solu- 



Cell. So in the Same simple tion of India ink. The matter discharged from 



. the pulsating vacuoles may be temporarily seen 



forms We have VaCUOleS forming as a n irregular area of clear fluid next to the 



Channels leading from the endo- bod y and surrounded by the darkened water. 



. i < i 11 (After JENNINGS.) 

 plasm to the exterior of the cell 



body. Because of their power of rhythmic contraction they are called 

 contracting vacuoles. Figure 301 shows that these vacuoles expel their 

 fluid contents. The fluid that the vacuoles constantly throw out in this 



339 



