Chapter 7 

 Major groups and phylogeny of Protozoa 



THE Protozoa are grouped into two subphyla: Plasmodroma (p. 

 198) and Ciliophora (p. 545). The Plasmodroma are more primi- 

 tive Protozoa and subdivided into three classes: Mastigophora 

 (p. 198), Sarcodina (p. 328), and Sporozoa (p. 427). The Ciliophora 

 possess more complex body organizations, and are divided into two 

 classes : Ciliata (p. 545) and Suctoria (p. 695). 



In classifying Protozoa, the natural system would be one which is 

 based upon the phylogenetic relationships among them in conform- 

 ity with the doctrine that the present day organisms have descended 

 from primitive ancestral forms through organic evolution. Unlike 

 Metazoa, the great majority of Protozoa now existing do not possess 

 skeletal structures, which condition also seemingly prevailed among 

 their ancestors, and when they die, they disintegrate and leave 

 nothing behind. The exceptions are Foraminifera (p. 394) and 

 Radiolaria (p. 417) which produce multiform varieties of skeletal 

 structures composed of inorganic substances and which are found 

 abundantly preserved as fossils in the earliest fossiliferous strata. 

 These fossils show clearly that the two classes of Sarcodina were 

 already w^ell-difTerentiated groups at the time of fossilization. The 

 sole information the palaeontological record reveals for our reference 

 is that the differentiation of the major groups of Protozoa must have 

 occurred in an extremely remote period of the earth history. There- 

 fore, consideration of phylogeny of Protozoa had to depend ex- 

 clusively upon the data obtained through morphological, physio- 

 logical, and developmental observations of the present-day forms. 



The older concept which found its advocates until the beginning 

 of the present century, holds that the Sarcodina are the most primi- 

 tive of Protozoa. It was supposed that at the very beginning of 

 the living world, there came into being undifferentiated mass of pro- 

 toplasm which later became differentiated into the nucleus and the 

 cytosome. The Sarcodina represented by amoebae and allied forms 

 do not have any further differentiation and lack a definite body 

 wall, they are, therefore, able to change body form by forming 

 pseudopodia. These pseudopodia are temporary cytoplasmic proc- 

 esses and formed or withdrawn freely, even in the more or less 

 permanent axopodia. On the other hand, flagella and cilia are per- 

 manent cell-organs possessing definite structural plans. Thus from 



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