MORPHOLOGY AND CLASSIFICATION 151 



The protozoan cell is generally larger and more complex in 

 structure than the bacterial cell appears to be, although the 

 viding line is in places indefinite or even wholly obscure. In 

 general the protozoon shows the typical structure of a single cell 

 of the metazoon. A well-defined nucleus is usually present, some- 

 times several of them, although in some forms the nuclear ma- 

 terial is more or less scattered throughout the cell. Most proto- 

 zoa exhibit differentiation of the protoplasm into cell organs or 

 organellae, adapted to perform certain functions. In many pro- 

 tozoa sexual reproduction has been observed, a process involving 

 complex morphological changes. The cells showing these evi- 

 dences of complex organization resemble in most respects cells 

 of the higher animals, and in fact a colony or group of protozoa 

 may be regarded as representing a transition to the many-celled 

 animals, just as, on the other hand, the bacteria were seen to be 

 connected with the higher plants through the forms of the higher 

 bacteria, the yeasts, the molds and algae. Physiologically, pro- 

 tozoa differ from bacteria and other plants in requiring more com- 

 plex nitrogenous food, but this distinction is far from absolute. 

 Doflein divides the protozoa into two substems, (i) Plasmodroma, 

 including those forms which move by means of pseudopodia or 

 flagella, and which exhibit for the most part an alternation of 

 asexual and sexual generations, and (2) Ciliophora, including 

 those forms which move by means of cilia and in which the sexual 

 fertilization gives rise to no special reproductive form of the 

 organism. 



The substem Plasmodroma includes three classes, (i) Masti- 

 gophora, (2) Rhizopoda and (3) Sporozoa. 



Flagellates. In the class Mastigophora, are included a great 

 many different organisms, the one common feature being the 

 type of locomotive apparatus, which consists of one or more fla- 

 gella. The further subdivision of the class has not yet been agreed 

 upon, not because of any lack of morphological differences upon 

 which to base a classification, but largely on account of difficulty 

 in estimating the relative importance and meaning of the many 



