132 PHYSIOLOGY 



which are given off by osmosis; and urea which is given 

 off in solution by the contractile vacuole. 



This organ is a true excretory organ, comparable to 

 the kidney of the higher forms. It is a clear space 

 which appears and disappears rhythmically, enlarging as 

 it becomes filled with fluid and contracting to eject 

 the fluid from the body. This organ must not be con- 

 fused with the anal spot. The excretory organ gives 

 off waste from the protoplasm itself, but from the anus 

 pass off those parts of the food which can not be made 

 into protoplasm and which therefore have never been 

 a part of the body of the animal. 



Respiration. — In one-celled animals the entire body 

 wall is surrounded by water which contains oxygen in 

 solution. By the process of osmosis oxygen passes into 

 the body at any point and carbon dioxide passes out. 

 This occurs not because the animal needs oxygen, but 

 because a gas passes in the direction of the least pres- 

 sure. The animal has no choice in the matter. 



Reproduction. — Two methods of reproduction exist. 

 The simplest of these is fission, a purely non-sexual 

 process by which an animal splits into two parts each 

 of which becomes a new animal. This process can not 

 continue indefinitely. It is supplemented by a sexual 

 process called conjugation. Two animals come close 

 together and exchange either the whole or a part of the 

 nucleus and separate. This brings about a rejuvenation 

 by which both animals regain the power of fission. 

 They are then able to divide and again divide until 

 enough cells are formed to make a well-differentiated 

 animal were they but joined together. As a rule con- 



