PROTOZOA AND BACTERIA 27 



bility, motility and reproduction. Most metazoan cells, on 

 the other hand, are so specialized for particular functions that, 

 if separated from the other cells with which they are associated 

 in the body, they die almost immediately. 



The very fact of evolution makes it difficult to draw a sharp 

 and fast line between two groups of organisms, even between 

 such fundamentally different groups as the Protozoa and Meta- 

 zoa. There are always border line exceptions which make the 

 work of the systematic zoologist at once difficult and interesting. 

 In the case in hand there are colonial Protozoa in which all of the 

 cells are not exactly alike, but have at least the beginnings of 

 specialization. Some protozoans, such as the intestinal flagel- 

 late Giardia (or Lamblia), are composed, as adults, of essentially 

 two cells instead of one. Such animals have been placed by 

 some authors in a distinct order to which the name Diplozoa 

 (double animals) has been applied. On the other hand, in the 

 lowest metazoans, the sponges, there is only very limited speciali- 

 zation of the cells, while in the little-known animals which are 

 designated as " Mesozoa " there is even less differentiation. 



The distinction between Protozoa and Bacteria, though in- 

 volving the distinction between animals and plants, is much 

 more difficult. As we descend the evolutionary scale of plants 

 and animals the usual distinctions between them disappear and 

 it becomes difficult if not impossible definitely to place certain 

 species in either the plant or animal kingdom. The possession 

 of a distinct nucleus of some kind and some type of sexual re- 

 production are the characteristics which usually distinguish the 

 Protozoa from the less highly organized Bacteria. Often, how- 

 ever, it is difficult to discover sexual phenomena, or to interpret 

 them with safety, and the presence or absence of a nucleus is 

 sometimes equally difficult to determine. In such cases pe- 

 culiarities in life cycle, chemical reactions, staining properties 

 and the like are resorted to as distinguishing characteristics. 

 Most biologists are now inclined to group all of the single-celled 

 animals and plants, including Bacteria, into one great group 

 known as the Protista, a suggestion first made by Ernest Haeckel. 

 The existence of such groups of organisms as the Spirochsetes 

 and the Piroplasmata, occupying intermediate positions between 

 Protozoa and Bacteria, and of such groups as the chlorophyll- 

 bearing flagellates, occupying an intermediate position between 



