10 PROTOZOAN PARASITISM 



These cavities are commonly called vacuoles. They 

 are essentially stomachs. 



The gastro-intestinal system of the protozoa is 

 therefore a remarkably adjustable one. 



In these vacuoles the usable nutriment is prepared 

 for absorption and metabolism. It is used or stored, 

 as the need may be, the storing of reserve materials 

 being apparently mainly a preparation for hiberna- 

 tion in the encysted phase, where sometimes multi- 

 plication takes place. 



The waste from digestion, and probably from the 

 completed metabolism, is expelled to the exterior. 

 Excretion thus takes place, commonly by rupture 

 of a vacuole containing waste material through some 

 part of the ectoplasm, sometimes through a special 

 or pore-like excretory aperture. 



The smallness of the bodies of these animals and 

 the accessibility of the protoplasm to the diffusion 

 of gases enables them to do without special respira- 

 tory tracts and systems, necessary to larger animals. 

 There are no recognizable structures directly con- 

 cerned with respiration. Parasitic protozoa, living 

 in the dark of the bodies of their hosts, possess no 

 chromotophores (color bearer) by which oxygen may 

 be separated from carbon dioxide of their mediums, 

 such as some free living forms exposed to light have. 



Internal metabolism by protozoa is carried on by 

 more or less constant movement of the cytoplasm in 

 currents. 



The structure of the inner substance (endoplasm) 



