32 



THE PROTOZOA 



Fig. 6. Naegleria gruberi. This protozoan can exist in either of two 

 forms: an amoeboid form or a flagellate form. Stages 1-8 show- 

 stages during which the amoeboid form changes into the flagellate 

 condition. The arrow indicates the direction in which the animal 



moves. (From Willmer.) 



stage. This too indicates that the flagellate stage is not necessarily 

 the more juvenile one. 



On the basis of this evidence we are left undecided as to which 

 is the most primitive, the Flagellata or the Rhizopoda. This 

 question will be dealt with again on p. 33, where a third inter- 

 pretation w r ill be presented. In effect this third view states that the 

 Rhizopoda and Flagellata are not strict classes of the Phylum 

 Protozoa. Instead they are polyphyletic grades. The Flagellata arose 

 on many separate occasions from the plants, fungi and metazoa, and 

 the Rhizopoda developed in much the same manner. Both these 

 groups are then more in the nature of horizontal grades than vertical 

 monophyletic classes, one of which is older than the other. 



Protozoan Phylogeny; the Interrelationship 

 of the Four Classes of Protozoa 



The precise relationship of the four classes of Protozoa is 

 uncertain. The two classes that appear to be the most closely 

 related are the Flagellata and the Rhizopoda. Butschli in 1883 



