128 MANUAL OF ELEMENTARY ZOOLOGY 



Protected by their horny cases, the sporozoites pass 

 through the gut of the bird and are distributed in its 

 droppings over the soil, where they are washed down by the 

 rain and presently swallowed by another worm with the earth 

 from which it obtains its food. The spore-cyst is dissolved 

 in the intestine of the worm, and the sporozoites come out 

 and bore their way through the wall of the gut and other 

 tissues till they reach the vesiculae seminales. Here each 

 enters a sperm-mother-cell, where it grows by absorbing the 

 protoplasm which is meant to serve for the nourishment of 

 the spermatozoa (see p. 228). The latter are formed, but 

 wither, their tails only remaining attached to the young 

 Monocystis, which looks as though it had a coat of cilia. 

 Finally they disappear, while the Monocystis continues to 

 grow. Thus the sporozoites become trophozoites by 

 development. 



In a later chapter (p. 151) there will be found 

 descriptions of animals related to Monocystis which are 

 parasitic in the body of man, where they are the cause of 

 malarial fevers. 



