SURVEY OF UNICELLULAR ANIMALS 47 



know what the biologist means by relationship. The answer is: 

 'blood relationship,' descent with change from a common an- 

 cestor — organic evolution. But this large subject must be left 

 until much later in our study; merely stating now that although 

 the object of a natural classification is to express the pedigrees of 

 the organisms, most classifications are, at best, still very far from 

 realizing this ideal. With probably a million known species of 

 animals on the Earth to-day, classification is a stupendous problem; 

 it is not a small problem even with the fifteen thousand known 

 species of Protozoa. For our purpose, however, classification can 

 be reduced to a minimum — to a mere skeleton outline, — to facili- 

 tate a synoptic view of the diversities of animals. (See page 352 

 and Fig. 313.) 



The first great subdivision, or phylum, of the Animal Kingdom 

 is the Protozoa. All of the Protozoa, since they are single cells, 

 demand for active life a more or less fluid medium, and are typi- 

 cally aquatic animals. However, different species exhibit all 

 gradations of adaptation to variations in moisture from those that 

 thrive in oceans and lakes, or pools and puddles, to those which 

 find sufficient the dew on soil or grass blade, or the fluids within 

 the tissues and cells of higher animals and plants. 



The phylum Protozoa is divided, largely on the basis of the 

 locomotor organs, into four chief groups, or classes: the Sar- 

 codina, Mastigophora, Sporozoa, and Infusoria. In general, we 

 may regard the Sarcodina as forms, like Amoeba, that move 

 about by means of pseudopodia; the Mastigophora as cells with 

 flagella as locomotive organs, such as Euglena ; and the Infusoria 

 as organisms, like Paramecium, that swim by cilia. The Sporozoa, 

 all of which are parasitic, such as the organisms causing malaria, 

 possess no characteristic type of organ for locomotion though 

 many are motile. (Figs. 13, 22, 27, 223.) 



A. Sarcodina 



Amoeba, which we have already studied, may be visualized as 

 the type of the Sarcodina since all members of this class are, 

 broadly speaking, 'amoebae,' but there are numerous species of 

 the genus Amoeba itself. The common, relatively large fresh- 

 water species, usually called Amoeba proteus, has as associates 

 many other fresh-water and some salt-water species. Again, numer- 



