GENERAL SKETCH 



47 



the Protozoa, although occasionally seen in some Metazoa (e.g. 

 Macrobiotus, or the " water bear," and some rotifers). This is a 

 special adaptive process by which the organisms are enabled to 

 survive when the environment is unsuitable. If a pool dries up, 

 becomes too dense, or too foul from putrefaction or other causes, 



Fig. 17. Types of cysts. 



A. Amceba proteus. [SCHEEL.] B. SfyloiyckiamytUusTS.hr. [BiJTSCHLl.] C. Pleurotricha 

 grandis St. [BiJTSCHLl.] D. Euglypha alveolata Dtij. [LEIDY.] E. Actmosphcerium Etch. Ehr. 

 [BUTSCHLI.] F. Colpoda Steinii. [MAUPAS.] a, gelatinous matrix ; b, outer cyst wall ; c, middle 

 cyst wall ; d, inner cyst wall. 



the cell draws in its appendages, rounds out into a sphere and secretes 

 a resisting membrane, within which it can exist for a long period. 

 When first formed, this membrane is a delicate gelatinous substance, 

 which soon hardens and gradually acquires the peculiar characters of 

 chitin. With the exception of the contractile vacuole, which continues 

 to contract rhythmically for some time, all of the organs of the body 

 are quiet at this period. The water, expelled by the vacuole, collects 

 between the cyst and the spherical wall of the animal, the latter 



