PROTOZOA 375 



The protozoa possess the functions of irritability, nutri- 

 tion, respiration and reproduction. They react in certain 

 characteristic ways towards stimuli of a chemical, mechanical 

 or electric nature. Protozoa receive nourishment by the in- 

 gestion or digestion of food and the excretion of effete matter. 

 Some species, especially the pathogenic forms, seem to absorb 

 food in the liquid form directly through the body wall. Most 

 of them, however, take solid food such as small animal or 

 vegetable organisms or bits of organic matter, through more 

 or less definite parts of the body. It is supposed that the con- 

 tractile vacuole which protozoa contain has a respiratory as 

 well as an excretory function. The interchange of gases is 

 constantly going on either through this vacuole or by osmosis 

 through a part of the cell wall. Protozoa reproduce themselves 

 through cell divisions and its various modifications. The 

 simplest variety is true cell division which may be either 

 longitudinal or transverse by either the direct or indirect 

 method. In some species there is a budding division which 

 consists in a small piece breaking off from the body and form- 

 ing a new individual. The reproduction by division or bud- 

 ding is interrupted at certain times in the history of each 

 organism. 



The development cycle of the protozoan consists in the 

 changes that take place in its growth from one act of fertiliza- 

 tion to another. 3 According to Calkins, the developmental 

 cycle as a whole should be considered the individual and 

 should be made the basis for the whole rather than for any 

 part or parts of it, for many protozoa pass through the sexual 

 part of their life cycle in one host and the asexual part in 

 another. 



Cultivation of Protozoa. Certain protozoa may be culti- 

 vated en masse in aquaria jars containing in the water bacteria 

 and the minute plants and animals usually kept in stock 

 aquaria. Pure cultures have recently been obtained of cer- 



3 In conjugation when union is transient or when union is per- 

 manent and likened to the process of the fecundation of the ovum. 



