CHAPTER TWENTY-FOUR 



PROTOZOA 



Characters i. Protozoa are usually defined as the simplest ani- 

 mals, consisting of only a single cell. Some forms, how- 

 ever, are said to be "colonial," existing in regular and 

 well-defined groups of numerous individuals or cells. 

 Thus the common fresh-water Anthophysa consists of 

 pear-shaped flagellate cells united in compact clusters, 

 often attached to a stalk. The slime molds or Myce- 

 tozoa form sporangia which are composed of many cells 

 and have the appearance of fungi. Even in these cases, 

 however, we do not find the development of tissues con- 

 sisting of specialized cells, such as exist in other groups 

 of animals. As if to make up for this lack of specializa- 

 tion, the single protozoan cell is often a remarkably 

 complex structure, having many recognizable parts, or 

 secreting an elaborately constructed shell. The Pro- 

 tozoa are readily distinguished from other animals if 

 attention is paid to their characters, though the smaller 

 worms may be confused with them on superficial ex- 

 amination. The latter, if examined more closely, will 

 be seen to have various complex internal organs wholly 

 lacking in Protozoa. When we look in another direc- 

 tion and try to separate the Protozoa from the Proto- 

 phyta or lowest plants, the task becomes much more 

 difficult. Indeed, many groups are claimed both by the 

 botanists and the zoologists. It might seem easy to 

 refer the green Euglena to the plants, since it possesses 

 chlorophyll, the characteristic coloring matter of green 

 plants. It is found, however, that very closely related 

 animals lack the green. There is another group which 

 makes a shell of cellulose, which is also a typical plant 

 product ; but in other respects the organisms resemble 



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