422 ZOOLOGY. 



actually destroy and digest the protoplasm of the host cell 

 is not certain. The formation of firm resistant cysts or 

 envelopes is also very characteristic of parasitic Protozoa. 



All parasites, both Protozoan and Metazoan, have 

 enormous powers of multiplication, without which they 

 would necessarily be in danger of extinction. For as 

 they are usually carried passively into the appropriate 

 host, and in any case depend upon chance for the oppor- 

 tunity of entering a host, it is necessary that there should 

 be an enormous number of spores or eggs to ensure that 

 some will survive. A large proportion of the spores or 

 eggs fail to find entrance into a host of the proper kind, 

 or are killed by unfavourable conditions. 



