32 CHEMICAL AND PHYSICAL PROPERTIES 



of fluids containing foodstuffs, and is therefore com- 

 parable to the wall of other vegetable and animal cells. 

 Protozoa. The chemical composition is probably 

 like that of bacteria, although little is known of it. 

 Their vital activities are influenced by physical con- 

 ditions, as is the case with all animate beings. They 

 require moisture for their full development, but may 

 live for indefinite times when it is at a minimum. A 

 definite temperature is demanded by each species or 

 genus for its full activity. They are susceptible to 

 high degrees and remain quiescent in nature in the cold 

 for a long time. Desiccation of the germinating forms 

 is usually fatal, but when in sporulation or encystment 

 drying is more easily withstood. Light is not abso- 

 lutely essential for the growth of protozoa, but they 

 are usually attracted or repelled by it; that is, few 

 if any are indifferent to luminosity. 



