PHYLUM PROTOZOA 21 



sents a mass of protoplasm which has undergone modifications 

 for varying functions though the body of the protoplasmic mass 

 has never become divided into cellular units. 



Some other workers prefer to retain the morphological compari- 

 son of a protozoan as a single cell while they draw their contrasts 

 on the side of differences in physiology or function. In his 

 "Biology of the Protozoa" Dr. Calkins has well expressed his 

 view in the words: 



. . . while a single protozoan is to be compared structurally with a single 

 isolated unit tissue cell of a metazoan as a bit of protoplasm differen- 

 tiated into cell body, or cytoplasm, and nucleus, it is a very different 

 unit physiologically. In its vital activities it should be compared not 

 with the unit tissue cells, but with the entire organism of which the 

 tissue cell is a part . . . it is still a cell and at the same time a complete 

 organism performing all of the fundamental vital activities within the 

 confines of that single cell. 



The complexity of structure within a single-celled protozoon is 

 well illustrated in the figure of Diplodinium which faces the title 

 page of this book. The multiplicity of specialized parts in this 

 animal does not permit one to think of it as a simple organism. 



Metazoan Tendencies. — There is no broad gap separating the 

 simple, single-celled Protozoa on the one side from the Metazoa 

 on the other. Many Protozoa, especially among the Mastigo- 

 phora and Ciliata, show distinct tendencies toward specialization 

 of cells. Frequently, the cells resulting from the division of a 

 single one remain attached, thus forming a group which functions 

 as a unit and is termed a colony (Fig. 5). Within such a group 

 only part of the cells may retain the powers of reproduction while 

 the others carry on all of the remaining functions for the colony. 

 This marks an early separation of two kinds of cells, the germ cells 

 as distinct from the body or somatic cells (Fig. 5). Separate germ 

 cells or gametes occur in many different kinds of Protozoa, in fact 

 it is not uncommon for two different kinds of gametes to make 

 their appearance in this group as microgametes (male germ cells) 

 and macrogametes (female germ cells), but in all these instances 

 all of the somatic cells remain similar. Thus, there is a finely 

 graded series of changes which connects the single-celled condi- 

 tion and the colony bearing only one kind of somatic cells and 

 one or more types of germ cells. Most zoologists agree that this 

 marks the limit of histological differentiation in the Protozoa. 



