38 



PROTOPLASM 



in appearance and habit from naked masses of protoplasm like 

 Amoeba, though all are one-celled organisms. 



Among the distinguishing characteristics of cells that are 

 unicellular organisms and those that are parts of tissues is the 

 ability of the former to live singly, while the latter are unable 

 to do so. As a result, the one-celled organisms are often much 

 more complex than are tissue cells and to such an extent that 

 while we are forced to recognize them as single cells, they cease 



al.o.V.t^^ 



mac 



cil.ec 



all 



A B 



Fig. 28. — A, Euplotes; B, diagram of Euplotes showing, Tnac. the macronucleus, 

 mic. the micronucleus, m. c. cirri, al. alveoli, cyt. the cytostome. 



to be so in the strict sense of the word. They are no longer just 

 cells but organisms as well. The protozoan Euplotes illustrates 

 this fact (Fig. 28). This one-celled animal lives in water, 

 as do most unicellular organisms. Euplotes is barely visible 

 to the naked eye as a tiny spot (the actual size is about 150 /x). 

 It is very active, for at various points protrude cilia used for 

 swimming. Some of these cilia are united to form stronger cirri 

 on which the organism can actually walk. The anterior portion 

 is mostly "mouth" into which food, consisting of smaller one- 

 celled plants and animals, is directed by means of other minute 

 cilia; there is also a posterior flange. The interior contains a 

 central region, or "stomach," a worm-shaped macronucleus, a 



