AMCEBA AND SOME ALLIES: PROTOZOA 291 



Unicellular organisms have long been objects of profound 

 study for biologists and psychologists. The biologists recog- 

 nize in them the morphological unit of structure, identical in 

 important details with the unit of structure in the tissues of 

 all animals as well as all plants, but differing from the tissue- 

 cells in the fact that the latter have only one chief function 

 to perform, whereas unicellular organisms carry out all the 

 processes of living in one small bit of protoplasm, the single 

 cell. Psychologists look to these organisms for the beginning 

 of that phase of mind called consciousness, one evidence of 

 which we see in the ability of ourselves and other animals to 

 choose a course of action. As a matter of fact, we have no 

 way of knowing whether unicellular organisms are conscious 

 or not. According to Professor Jennings, these organisms, 

 as well as others, manifest complicated internal physiological 

 processes. Any external agents, as, for example, oxygen, heat, 

 or food, which bear relation to the internal physiological 

 processes, are concerned in the activities of the organism. If 

 a Paramoecium comes to a place where oxygen is scarce, it 

 will make a series of trial trips in various directions, until the 

 internal demand for oxygen is satisfied. As the result of 

 the needs of the organism, it is constantly on the move, but 

 we cannot say that these movements are conscious efforts. 



Definition of Protozoa (Gr. protos, first ; zoon, animal). 

 The four members of the phylum described in this chapter 

 were selected as representatives of the four classes that make 

 up the phylum : Amoeba proteus, of the class Sarcodi'na ; 

 Euglena viridis, class MastigopJi' ora ; Plasmodium malarise, 

 class Sporozo'a; and Paramoecium caudatum, class Infuso'ria. 



Although in each class the genera differ widely, all the 

 members of the four classes agree in being composed of 

 single cells. Nearly all species are microscopic in size. 



Reproduction is brought about by division of the cell, and 

 never by eggs and spermatozoa. 



