OF THE ALIMENTARY TRACT 3 



and not interchangeable from one kind of animal 

 host to another. 



The bodies of the active form of protozoa are naked 

 and their locomotor apparatuses are but primitive. 

 They are therefore largely dependent upon a liquid 

 medium for their activities of movement and search 

 for food and for protection from evaporation of the 

 water of their own bodies. Some are dependent upon 

 food in solution, also. Consequently the necessity of 

 a fluid environment for them. Those which are not 

 subject to periodic drying of their habitat and which 

 do not need protection in passing from one host to 

 another exist mainly in the naked active form and 

 do not encyst. 



Various temperatures of their fluid mediums are 

 suitable to the various species. Activity of free 

 living species goes on under considerable natural tem- 

 perature variation. Activity in those parasitic in 

 cold blooded animals has an optimum temperature 

 corresponding to that of the host. In the protozoa 

 of warm blooded animals warmth of the range of the 

 host is the optimum state for activity and they usually 

 cease motion and soon die upon cooling, even for a few 

 degrees. 



Naturally the naked forms are very susceptible to 

 heat and the cysts are also killed at a definite and not 

 high degree, as is known for some parasitic species. 

 While cold readily kills the active protozoa of warm 

 blooded.animals, the free living and those of the cold 

 blooded may survive and may even be active short of 



