Ill PLATODES— SEXUAL ORGANS 155 



the Trematoda, are hermaphrodite. The male sexual products, however, 

 almost universally develop before the female — a phenomenon which 

 has received the name of protandrous hermaphroditism. Both male 

 and female sexual apparatus consist (1) of places of formation of the 

 sexual products which are imbedded in the body parenchyma (ovaries 

 and testes), (2) of special canals, efferent duets, which conduct the 

 sexual products away from the places where they were formed to (3) 

 the outer eopulatory apparatus. "We will describe these three parts 

 in succession. 



A. The Places of Formation of the Sexual Products. 



1. The Female Germ Glands. — We meet with these in the Platodes 

 in two forms. First, and this is the simpler and no doubt also the older 

 condition, as simple ovaries, in which the egg germs rijDen into eggs, in 

 whose protoplasm particles of deutoplasm or nutritive yolk occur. 

 Secondly, and this is the derived condition, in the double form of 

 germaria and vitellaria. The germaria yield the egg germs, i.e. 

 the young egg cells. The vitellaria, however, have undertaken the 

 work of supplying these egg cells with the nutritive yolk which is 

 necessary for their further development. Comparative research has 

 shown that the vitellaria are not newly formed accessory glands of the 

 female sexual apparatus, but that they are metamorphosed ovaries 

 or portions of ovaries adapted to a special function. It need not be 

 pointed out that the germaria are ovaries. 



In the Polyclacla there are only ovaries and no vitellaria. The 

 ovaries (Fig. 24, D, p. 29) are roundish bodies whose structure exactly 

 corresponds with that of the female gonades of the higher Cnidaria. 

 They lie in great numbers (Fig. Ill, o) on and between the intestinal 

 branches or gastro-canals in the lateral parts of the body. 



In the Triclada, besides a few eggs in the discharged egg cocoons, 

 there are extraordinarily numerous yolk cells, which serve to nourish 

 the former. Light is thrown on this phenomenon by the fact that 

 in certain Polydada and MoUusca several eggs may be deposited in 

 one cocoon, of which, however, only some develop; the others 

 sooner or later become disorganised and serve as food for the 

 former. Thus perhaps the yolk cells in the Triclada cocoon may be 

 considered as modified egg cells, which develop no further, but serve 

 as food for the few fertilised egg cells which do develop. Division 

 of labour, therefore, has stepped in among the germ-preparing organs, 

 the ovaries ; some yield eggs capable of fertilisation and development ; 

 others yield modified egg cells laden with yolk, which serve as food 

 for the above, and are no longer capable of fertilisation and develop- 

 ment. The first are the germaria (Fig. 112, ks), the second the vitellaria 

 (ds). These two are homologous structures, and in a young condition 

 look quite alike. In consequence of the large number of yolk cells 

 which are given to the eggs, the vitellaria are far more numerous than 



