GERM CELLS OF HERMAPHRODITES 191 



cious or gonochoristic. In the other group both 

 male and female reproductive organs occur in each 

 individual ; and such species are called monoecious 

 or hermaphroditic. The majority of animals are 

 gonochoristic, but a number of classes and orders 

 consist almost entirely of hermaphroditic species, 

 and probably no large group of animals is free from 

 species which are monoecious. A study of hermaph- 

 roditism is necessary for the elucidation of many 

 biological problems; and some of those dealing 

 more directly with the germ-cell cycle will be con- 

 sidered in this chapter. 



There are many variations in the morphology of 

 the reproductive organs in hermaphrodites. In 

 some, such as the earthworm (Fig. 56), the male 

 and female organs, consisting of all the parts typically 

 present in gonochoristic animals, are present and 

 entirely separate from each other. All gradations 

 between such a state and an intimate association of 

 male and female germ cells are known. Perhaps the 

 most interesting series occurs among the mollusks. 

 Here the germ gland may consist of two regions, as 

 in Pecten maximus, one of which gives rise to ova, the 

 other to spermatozoa ; or certain cysts may contain 

 only female germ cells and other cysts only male 

 germ cells, or both sorts of germ cells may occur in a 

 single cyst. 



Hermaphroditism has been shown to be prevalent 

 among animals that are parasitic or sedentary, or for 

 some other reason may become isolated from their 

 fellows. Thus, it is of advantage for a parasite, such 



