REPRODUCTION 



143 



Fig. 67. Encystment of Exiglypha acanthophora, X320 (Klihn). 



live. In some cases, the organisms encyst temporarily in order to 

 undergo nuclear reorganization and multiplication. Because of 

 the latter condition and also of the failure in attempting to cause 

 certain Protozoa to encyst under experimental conditions, some 

 suppose that certain internal factors play as great a part as do 

 the external conditions in the phenomenon of encystment. 

 Ordinarily a single cyst wall seems to be sufficient to protect the 

 protoplasm against unfavorable external conditions. In some 

 cases there may be a double cyst wall, the inner one usually being 

 more delicate. The cyst wall is generally composed of homo- 

 geneous substances, but it may contain calcareous scales as in 

 Euglypha (Fig. 67). While chitin is the common material of which 

 the cyst wall is composed, cellulose makes up the cyst envelope 

 of numerous Phytomastigina. 



The capacity of Protozoa to produce the cyst is probably one 

 of the reasons why they are so widely distributed over the surface 

 of the globe. The minute protozoan cysts are easily carried from 

 place to place by wind, attached to soil particles, debris, etc., by 

 the flowing water of rivers or the current in oceans or by insects, 

 birds, other animals to which they become readily attached. 

 When a cyst encounters a proper environment, the living proto- 

 plasmic contents excyst and the emerged organism once more 

 return to its active trophic phase of existence. 



In Sporozoa, no encystment occurs. Here at the end of active 

 schizogony, sexual reproduction usually initiates the production 

 of large numbers of the spores (Fig. 68). 



Sexual reproduction and life-cycles 



Besides reproducing by the asexual method, numerous Proto- 

 zoa reproduce themselves in a manner comparable with the 



