20 SYSTEMATIC ZOOLOGY 



impracticable in a unicellular organism, though even here 

 certain parts of the cell are differentiated or set apart for the per- 

 formance of definite functions. Thus we find in almost all the 

 Protozoa a nucleus or nuclei, and even in the few species in which 

 no nucleus has as yet been observed it is very probable that nu- 

 clear material is present in some form ; the nucleus not only is 

 essential to the life of the individual, but plays the initial and 

 chief role in the process of reproduction. The earlier investiga- 

 tors, working with their imperfect microscopes, naturally formed 

 an erroneoo's conception of the structure of the Protozoa, and 

 even as late as 1838 assigned them a complete series of organs 

 corresponding to the functions of the animal body, — a nervous 

 system, a digestive system, etc. 



The classification of the Protozoa is based for the most part 

 on structural, i.e. anatomical, differences, which are due chiefly to 

 different methods of locomotion and nutrition. Four classes are 

 generally recognized. 



CLASS I. RHIZOPODA 



The Rhizopoda (Gr. pi£a, root, and 7rou?, foot), sometimes 

 called the Sarcodina (Gr. erap/«i>5>/?, fleshlike), are found in 

 fresh and in salt water, and are distinguished from the other 

 groups of Protozoa by the absence of any external membrane 

 to limit the shape of the body. Nevertheless, most of the mem- 

 bers of this class have a pretty definite form, which is due to the 

 presence of a skeletal structure, either external, covering more or 

 less of the outside of the body, or internal within the protoplasm. 

 All the Rhizopoda possess the property of protruding from the 

 surface of their bodies rootlike, filamentous, or threadlike 

 processes of the protoplasm, or sarcode as it is often called 

 here, of which they are composed. These processes are 

 known as pseudopodia, false feet ; for, though thev constitute 

 the organs of locomotion, they are not permanent structures, 

 but may in most cases be protruded from any point on the 

 periphery of the organism and then retracted. In those 

 species, however, where there is an external skeleton with 

 but one or two openings the pseudopodia can obviouslv be 

 extended through these openings only. The protoplasm of 

 the Rhizopoda is generally differentiated into two regions, 



