54 ELEMENTS OF BIOLOGY 



of animal life. The illustration of certain fundamentals is still 

 clearer in the life processes of some of the single cell plants, the 

 PROTOPHYTA. Morcovcr, the lives of some of these minute plant and 

 animal forms are passed in the tissues of the human body; hence 

 they are of great interest because they profoundly affect human 

 welfare. 



While the Protozoa are often spoken of as the simplest of ani- 

 mals, it must not be understood that the body of a protozoon is a 

 simple type of system; on the contrary, the actual physico-chemical 

 processes that are involved in protozoon life are not as well under- 

 stood as are those of the human body, or at least no better under- 

 stood. The simplicity of the Protozoa rests solely on the fact that 

 their bodies are single cells. Vital processes are demonstrated in 

 their most primitive aspects, and observations are simplified. • 



Types of Protozoa. Four classes of Protozoa are recognized, 

 the basis of distinction being, for our purposes here, largely their 

 differences in methods and organs of locomotion (Figs. 28, 29, 30, 

 31). Members of one class, called the sarcodina, move by means 

 of temporary extrusions of the cell, the cell substance flowing out 

 into these extrusions and withdrawing elsewhere. These temporary 

 organs of locomotion are known as pseudopodia. There are a great 

 many different Protozoa that move about in this fashion. For con- 

 venience the biologist divides the class on the basis of other resem- 

 blances into subordinate groups that are in turn still further di- 

 vided into groups termed genera. A genus is further divided into 

 smaller groups, within which the animals are almost identical, 

 known as species. The genus Amceba is the most common repre- 

 sentative of the class Sarcodina and the species "proteus" is usually 

 regarded as representing a typical member of the class. Animals 

 are ordinarily designated in terms of their genus and species, the 

 name of the genus being capitalized, for example, Amoeba proteus; 

 Rana pipiens (common frog); Hofno sapiens (Man). 



The second class of Protozoa includes those forms that have a 



