Reproduction and Life-Cycles 73 



different in appearance and behavior. Examples include the ciliated larva 

 and non-ciliated adult of Suctorea and the stalkless telotroch and the 

 stalked adult of vorticellid ciliates. Dimorphism sometimes involves the 

 alternation of amoeboid and flagellate stages (Fig. 2. 14, C-E; H-J). 

 The flagellate stage may be temporary, as in Naegleria; or it may be the 

 dominant stage, as in Tetramitus and certain Chrysomonadida. The flag- 

 ellate, Gigantomonas herculea, shows amoeboid-flagellate dimorphism in 

 which reproduction is limited to the amoeboid phase. Reproductive stages 

 in Haematococciis and related genera also are typically non-flagellated. 

 The dominant phase in Colaciiim (134) is a non-flagellated form which 

 occasionally produces flagellate buds (Fig. 2. 14, A, B). Dimorphism also 

 may involve the alternation of a gamete-producing stage and one which 

 undergoes asexual reproduction, as in Foraminiferida. Life-cycles char- 

 acterized by more than two active stages are found in certain Trypano- 

 somidae, in many Sporozoa, and in some of the Ciliophora. 



Protozoan life-cycles may be considered adaptive in that they represent 

 responses to changes in the environment, and perhaps favor, or insure 

 survival when such changes occur. Occurrence of a cycle as such probably 

 is dependent directly upon the environment. This seems evident in 

 parasitic species which must reach a susceptible host in order to complete 

 the cycle, or in many instances even to survive for more than a short time. 

 Within a suitable host, there is often reasonable security during comple- 

 tion of a life-cycle, but establishment in a host does not necessarily insure 

 independence of external conditions. For example, the development of 

 Plasmodium vivax in the mosquito may be retarded or prevented by un- 

 favorable temperatures. A modification of environmental conditions may 

 induce a marked change in the cycle, in parasitic as well as free-living 

 species. Maintenance of Plasmodium- gallinaceum in chick-tissvie cultures 

 has caused a normal stock to lose its ability to produce pigmented ery- 

 throcytic stages. Chicks inoculated from such cultures always died from 

 exoerythrocytic infections, always without showing normally pigmented 

 erythrocytic stages, and often without any erythrocytic parasites at all 

 (160). In some cases it has been possible to eliminate cyclic changes by 

 strict control of environmental conditions, as in the prevention of con- 

 jugation and encystment in ciliates by Woodruff, Beers, and others. Such 

 elimination of cyclic changes does not necessarily mean that the particular 

 life-cycles have no significance. Since a given cycle presumably adapts a 

 species to changing environments it may normally encounter, a perfectly 

 uniform environment may fail to evoke the cycle. 



Cysts 



Encysted stages, in which the organism is enclosed Avithin a cyst 

 membrane, are a common feature of protozoan life-cycles. On rhe basis 



