CHAPTER VI 



LIFE CYCLES: EFFECTS OF PARASITISM ON LIFE CYCLE, 

 BINARY AND MULTIPLE FISSION, ENCYST:\[ENT, SEX 



The question of pohTnorphism is one ever present in dealing not only with 

 the Protozoa but with most of the lo\Yer iuvertel^rates as well. Protozoology 

 as a science is still so new and its accumulated data, though already vast and 

 daily increasing, so vague in outline and so interwoven with error, consequent 

 on its very ne'svness, that this and related fundamental questions can only be 

 touched upon lightly, with the full knowledge that conclusions reached today 

 nmst wait upon further investigation for confirmation or refutation. That 

 this nmst be so does not conflict with the need for summarizing present achieve- 

 ment in any field, nor the formulation of conclusions drawn from existing data. 

 Indeed, these serve only to give a fresh impetus to investigation and to indicate 

 the lines along which important results may be found. 



The possible connection between length of period of evolution and the 

 complexity of the life cycle resulting therefrom finds some striking confirmation 

 in the Protozoa as compared with the ^Metazoa, in which pol^^uorphism tends 

 generally to disappear among the later evolved and more highly specialized 

 groups, except as related to sex. That other factors may enter into a consid- 

 eration of this prol)lem does not alter the fact that pol}^norphism occurs in 

 inverse i-atio to the length of time of evolution of the group, subject to secondary 

 modifications associated with adaptive processes as a result of parasitic and 

 social life and considerable changes of habitat. 



In only a few forms among the Protozoa has a complete life cycle been 

 followed through from its initial zygote to the beginning of the next cycle. This 

 has resulted in the rather general conclusion that the life cycle is simple in all 

 but a few groups, the different stages being simmiarized in the two processes 

 of growth and binary fission. There is almost no positive evidence that this 

 is true for a single protozoan. On the contrary, it is becoming increasingly 

 evident that a much more complex state of affairs exists. 



This question of pohTnorphism has a very direct bearing on any critical 

 study of the dinoflagellates, since one family of the Gymnodinioidae, the Pyro- 

 cystidae, has been foimed for what later evidence clearly indicates are not 

 generically distinct organisms. This organism is the large, globular form, 

 Pyrocystis, which breaks up into sickle-shaped cysts (fig. I), the contents of 

 which divide again to fonn small, Gynmodiitiion-like flagellates, usually eight 

 iu number. Evidence is accumulating, some of it still unpublished work by 

 the senior author, which shows that such a stage is an integral part of the life 

 cycle of genera as different as Gymuodinium and Gonyaulax. It is probable 



[62] 



