CHAPTER 4 



THE PROTOZOA 



We have just seen that the relationship between the simplest 

 living forms, the Viruses, Rickettsiae and Bacteria, is not at all 

 clear. We cannot say with any certainty how they have evolved 

 and what the relationship is between the three groups. When we 

 come to consider the next group of animals, the Protozoa, we shall 

 find a very similar situation. There is great difficulty in deciding 

 where the Protozoa came from, what they gave rise to, and what 

 their interrelationships are. The Protozoa can be classified into 

 four classes. These are: 



(1) Flagellata; e.g. Chlamydomonas, Trichonympha. 



(2) Rhizopoda; e.g. Amoeba, Elphidium. 



(3) Sporozoa; e.g. Monocystis, Plasmodium. 



(4) Ciliophora; e.g. Paramecium, Entodinium. 



There are several evolutionary problems to be found in the 

 protozoans but only three of these will be considered here. The 

 first problem is, " Which of the four classes is the most primitive? ' 

 The second problem is, " What is the interrelationship of the four 

 classes? " and the third problem is, " What is the status of the 

 group Protozoa? ' These problems will each be examined in turn. 



The Most Primitive Protozoa 



We can readily dismiss two of the four classes of the Protozoa 

 as candidates for the position of the most primitive class. The 

 Sporozoa are almost entirely parasitic in the bodies of higher 

 animals and they spend their life in the outside world in the 

 encysted state. Though it is possible that they could have arisen 

 in the " primaeval soup " that gave rise to living forms, their life 



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