GENERATIVE ORGANS OF MOLLUSC A. 381 



the ova, and those which produce the semen, are united together, and 

 the efferent ducts are common to both sets of products. In Pecten 

 also (P. varius) the latter arrangement obtains, but the germ- 

 gland itself is differentiated into a male and a female portion. The 

 former is placed in front and above, the latter behind and below. 

 When, finally, as in other forms, the separate germ-glands have 

 ducts which open separately, they are more highly differentiated 

 (Pandora). In many genera hermaphroditism predominates in some 

 species, while others have the sexes separate (Cardium). 



The efferent ducts of the germ-glands arc feebly developed, 

 and the glandular lobules are often placed close to their common 

 orifice. There are, therefore, no accessory organs of any kind. 

 They open in varions ways on either side. The genital canal is 

 sometimes united with the excretory organ, so that it looks like a 

 differentiation from it, and the generative products are passed out 

 through it (e.g. Pecten, Lima, Spondylus) ; the genital canal is 

 sometimes united with the orifice only of this organ (e.g. Area, 

 Mytilus, Pinna) ; lastly, it sometimes opens by a proper papilla 

 (e.g. in Ostrea, Unio, Anodonta, Mactra, Chama). 



From these facts it is clear that the excretory apparatus is of 

 great importance in aiding in the formation of the efferent ducts of 

 the generative products. The genital canal, inasmuch as it opens into 

 the excretory organ, appears to be a differentiation of the latter, 

 which is extended to the germ-glands in which the generative pro- 

 ducts are developed; and the gradual separation of the genital canal 

 from the excretory organ implies a further stage of differentiation, 

 which leads to a complete separation of the genital canal, and con- 

 sequently of the generative organs, from the excretory organ. This 

 arrangement, which is seen in all the higher Mollusca, 

 must therefore be derived from a primitive functional con- 

 nection between the generative and excretory organs; this 

 relation is, later on, simply indicated in a faint manner by the 

 approximation of the external openings of these organs. 



The Lamellibranchiata, by showing the lines along which the 

 differentiation of the efferent ducts of the generative organs has 

 been effected, are not, so far as these relations are concerned, so 

 very distant from the Vermes or the Brachiopoda. Some of the 

 Vermes present us with relations identical with those of the Lamelli- 

 branchs; while others, having the efferent apparatus greatly, and 

 apparently specially, complicated (Platyhelminthes), make it at first 

 sight more difficult to give any explanation of this matter. 



§ 294. 



The generative organs of the Gastropoda and Pteropoda are in 

 many ways more highly differentiated. Although there is a " her- 

 maphrodite gland," as in the Lamellibranchiata, which is found in 

 a larger number of forms, the apparatus is considerably compli- 

 cated, and, as a rule, has copulatory organs connected with it. The 



