THE PROTOZOA 35 



polyphyletic. . . . The Sporozoa are again a heterogeneous group of 

 which the different orders have probably had separate origins. . . . 

 The Ciliata differ so markedly from the other Protozoa in their 

 possession of cilia, nuclear dimorphism, and sexual phenomena 

 that their relation to them remains problematical." 



So of the four classes of the Protozoa we see that at least three 

 are suggested by Hyman as being polyphyletic. 



Baker (1948) has similar doubts about the status of the Protozoa. 

 In particular he considers the relationship of the din ofl age 11 ate 

 Gymnodinium with the filamentous alga Dinoclonium (Fig. 7). 

 During the life cycle of Dinoclonium it develops spores almost 

 indistinguishable in structure from Gymnodinium, but Dinoclonium 

 is placed in the Algae whilst Gymnodinium and Amoeba are placed 

 in the Protozoa. The structure of these spores clearly shows that 

 Gymnodinium is more closely related to Dinoclonium than it is to 

 Amoeba. Baker concludes that the Protozoa cannot be a mono- 

 phyletic group. 



From the evolutionary point of view we therefore have several 

 problems in the Protozoa. 



(1) The Protozoa do not seem to be a group of closely related 

 animals. It is most likely that they are a polyphyletic group and the 

 name " Protozoa " indicates a grade or status rather than a 

 natural taxonomic group. In this they would be analogous to the 

 group " Vermes " or " Pisces " ; i.e. they show a level of organisa- 

 tion and not an evolutionary relationship. (We shall see that this 

 problem arises again and again; many of our phyla and classes are 

 grades of animals that are not closely related.) 



(2) It is difficult to decide which of the Protozoa are the most 

 primitive. The information at our disposal is not sufficient to 

 allow us to come to any definite conclusion. 



(3) Each of the four classes probably contains the results of 

 convergent development from heterogeneous stocks. 



4— IOE 



