INVERTEBRATA. 



415 



worm to the spermathecae of another. But the spores 

 have never been seen in the spermathecae or in the cocoons. 

 It seems probable, therefore, that the spores are only set 

 free when the host worm is killed and destroyed, as, for 

 instance, when the worm is devoured by a bird. Pfeiffer 

 has actually observed the spores in the contents of the 

 intestines of various birds. In this case the spores would 

 pass with the excrement of birds into the soil, and there be 

 swallowed by other earthworms. The sporocyst is then 

 probably dissolved by the digestive secretions of the worm, 

 and the sporozoites set free, being actively motile, traverse 

 the tissues and reach the seminal vesicles. 



6. Development of Sporozoite and Trophozoite. The 

 earliest known stage in the development of the parasite 

 is a minute nucleated body contained in the central residual 

 protoplasm of one of the sperm morulae (fig. 205, A). The 

 parasite is, therefore, in its earlier stages intracellular, like 

 many other sporozoa. The sporozoite seems, therefore, 

 to attack one of the male germ cells, and to enter it, re- 

 maining in it when it divides and develops into sperma- 

 tozoa. This is called the trophic or feeding stage, and the 

 individual is called a trophozoite ; it is nourished by the 

 protoplasm in which it lives. It grows at the expense of 

 the residual protoplasm of the sperm morula until that 

 protoplasm is all consumed, and then for a time the 

 trophozoite is found surrounded with the spermatozoa of 

 the earthworm which have developed around it, so that it 

 might be mistaken for a ciliated organism (fig. 205, B). 

 Ultimately, these spermatozoa are detached and the 

 trophozoite becomes free, and thus we reach the stage 

 with which we began the description of the life-cycle. 



7. Life-history Formula. The life-history may be 

 briefly exhibited by the following symbolic formula : 



Sporozoite -> trophozoite -> gametocyte -> n gametes ") 

 Sporozoite -> trophozoite -> gametocyte -> n gametes ) 



-f = n zygotes -> n spores X 8" sporozoites, 

 which is read thus : The sporozoite becomes a trophozoite 

 which becomes a gametocyte, which segments into a 



