CHAPTER XI 



THE LIFE CYCLE OF THE PROTOZOA 



Charles Atwood Kofoid 



Introduction 



The organism has the fourth dimension of time. In the course of its 

 life cycle, its three spatial dimensions change. The fourth changes also, 

 interacting with the three. It may be measured by metabolic rate, by struc- 

 tural results of growth, or by organismal cyclic changes which follow 

 one after the other in sequences. These may be regular, interrupted, 

 repeated, or in some other way responsive to or dependent upon internal 

 environmental conditions, or to external conditions, such as changing 

 quantity or quality of food supply; rise or fall in temperature, of 

 seasonal origin, or due to migration; inciting or deterrent chemical or 

 physical factors, such as pH, intensity and duration of light, and changes 

 of host. 



The Protozoa* differ from the Metazoa because of their smaller size 

 and the resulting more highly significant and potent surface-volume re- 

 lations, as these affect the rate and intensity of the impact of environ- 

 mental factors upon the organism and the changes they initiate and 

 induce. It is therefore to be expected that the Protozoa will be relatively 

 more susceptible to the modification of the individual and to the dis- 

 tortion and interruption of its normal life cycle than are the Metazoa, 

 thus obscuring and complicating the evidence of the existence of life 

 cycles among them. The factors of time, volume, and season enter more 

 or less definitely into the life cycles of Metazoa such as hydroids, flukes, 

 tapeworms, crustaceans, insects, and tunicates. Among the Protozoa, on 

 the other hand, the time units required for the various cyclic changes 

 may be very brief, and these changes very often have little or no de- 

 pendence upon cosmic cycles, with the result that the evidence of their 



* Assistance in preparing this chapter, rendered by the personnel of Work Projects 

 Administration Official Project No. 65-1-08-113, Unit Cl, is acknowledged. 



