THE SPOROZOA 171 



infect whole tracts of tissue cells, causing serious illness and 

 even the death of the infected animal. In some Sporozoa, 

 also, one portion of the life cycle of the parasite is passed in 

 one host, and the other portion in another host of a quite 

 different kind, this second or intermediate host serving as 

 a carrier of infection from one primary host to another. In 

 such cases considerable complications are introduced into the 

 life cycle, as will be explained more fully in the next chapter. 



