PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 97 



firm and membrane-like upon continued contact with the water, and, 

 finally, if conditions continue unsuitable, it turns into chitin, which 

 withstands drought or heat, and within it the reduced sphere of pro- 

 toplasm is protected until conditions are again favorable. The 

 chitin is then reduced or dissolved by enzymes from within the cell, 

 or by external agents acting on it, and the organism creeps out and 



Fig. 35 



Gonium pectorale in reproduction. Eacli of tlie sixteen ceils of tlie coiony is dividing 

 to form a daughter colony of si.xteen cells. (.After Calkins.) 



resumes active life. Within such protecting cysts many different 

 types of protozoa go through the often complicated processes of spore 

 formation. In some cases the protection seems to be hardly neces- 

 sary, and spores are formed and liberated before the membrane has 

 had an opportunity to harden. This is the case in colpidium and in 

 Tillina magna, for example; in colpitlium, four or eight daughter cells 

 7 



