THE FLAGELLATA 205 



this is a very common phenomenon amongst the Protozoa, 

 yet it cannot be called a universal one, since we have been 

 unable to record any exhaustion following upon repeated 

 binary fission, either in Amoeba or in Euglena. In neither of 

 these forms does it appear to be necessary that at regularly 

 recurring periods individuals should conjugate to form zygotes 

 which, after a period of rest, enter upon a new lease of repro- 

 ductive life. Yet the phenomenon of conjugation is so 

 common both in the lower plants and animals that we are 

 tempted to believe that it is a very necessary condition of 

 existence and that it has simply been missed, that we have 

 failed to observe it, in the forms in which it is not recorded. 

 We have seen that it plays a very important part in the life 

 cycles of Monocystis and Plasmodium, that the nuclei of the 

 cysts of Actinosphaerium conjugate in a very peculiar way in 

 couples, and that there is a somewhat similar fusion of the nuclei 

 in couples in the Mycetozoa. In some of these forms the con- 

 jugating individuals or gametes are perfectly similar, but in 

 others they exhibit a more or less marked dissimilarity and 

 forecast in a greater or less degree the differentiation of sex 

 which is so characteristic both of the higher animals and plants. 

 Polytoma uvella is colourless and devoid of chlorophyll, 

 but the majority of its nearest allies are green, having one or 

 more chromatophors containing chlorophyll. Their nutrition 

 is holophytic, and they are so plant-like in structure and 

 in function that they are always described as plants in 

 botanical works. Yet the zoologist, recognising their affinities 

 to Flagellata, whose behaviour is clearly animal, cannot afford 

 to leave them out of consideration, and hence they are 

 classed in zoological works under the name Phytomastigoda, 

 or plant-like Flagellates. Some of them are of so much interest 

 and importance that they must engage our attention for a 

 while. 



