32 



EVOLUTION AND ANIMAL LIFE 



if such protoplasmic cells, composed of nucleus and cytoplasm, 

 exist singly they form living units. And we have actual ex- 

 emplifications of this condition in the structure and life of the 

 simplest organism. 



The simplest organisms are independently living, single proto- 

 plasmic cells (Figs. 16-21). There are thousands of kinds of 



CJik IT H 



FIG. 19. Plasmodium of a slime mold on wood, Trichia fai-aginea: A, plasmodium X 2; 

 B, spores; C, spore with contents escaping; D, ciliated swarm spore, showing 

 flagellum, /, and nucleus, n; E, two amoeboid swarm spores; F, part of plasmodium 

 under glass slide; G, a part of F, more highly magnified. (After Campbell.) 



these single-celled organisms recognizably different by charac- 

 teristics of shape and size, habit and habitat. We try to distin- 

 guish them as single-celled animals (Protozoa) and single-celled 

 plants (Protophyta) , on the basis of alleged differences in their 

 habit of food-taking and general nutrition. This distinction is 

 often most arbitrarily made, and botanists and zoologists are 

 constantly claiming the same organisms as belonging to their 

 respective fields of study. Many naturalists, conspicuously 

 Haeckel, have repeatedly suggested the convenience and even 

 the necessity of grouping most of these unicellular organisms 

 into a phylum or kingdom to be called the Protista, the members 



