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 moncecious 
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 moneecious. 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 
