134 TYPES OF SEXUAL DIFFERENCE chap. 



2. The sexes are united in 



Gasteropoda Opisth ohranchiata. 

 Gasteropoda Pulnfionata. 

 Certain Pelecypoda} 



In the dioecious MoUusca, sexual union is the rule, but is by 

 no means universal. In some instances, — e.g. Vermetus, Magilus, 

 Patella, Haliotis, Crepidula, Chiton, the Scaphopoda — the form 

 and habits of the animal do not admit of it ; in others (many 

 Trochus) a male copulative organ is wanting. When this is the 

 case, the male scatters the spermatozoa freely ; the majority must 

 perish, but some will be carried by currents in the direction of the 

 female. 



When the sexes are separate, the female is frequently larger 

 than the male. This is markedly the case in Littorina, Buccinum, 

 and all the Cephalopoda ; in Argonauta the difference is extreme, 

 the male not being more than ^ the size of the female. 



Those hermaphrodite Mollusca which are capable of sexual 

 union (Gasteropoda, Pulmonata and Opisthobranchiata) are con- 

 veniently divided into two sections, according as (1) there are 

 separate orifices for the male and female organs, or (2) one orifice 

 serves for both. To the former section {Digonopora ^) belong the 

 Limnaeidae, Vaginulidae, and Onchidiidae, and many Opistho- 

 branchiata, including all the Pteropoda ; to the latter {Mono- 

 gonopora 2) nearly all the Nudibranchiate Opisthobranchiata, and 

 all the rest of the Pulmonata. In the latter case during union, 

 mutual impregnation takes place, and each of the two individuals 

 concerned has been observed (compare p. 42) to deposit eggs. In 

 the former, however, no such reciprocal act can take place, but 

 the same individual can play the part of male to one and female 

 to another, and we sometimes find a string of Limnaea thus 

 united, each being at once male and female to its two adjacent 

 neighbours. 



The Reproductive System. — Broadly speaking, the compli- 

 cated arrangements which are found in Mollusca resolve themselves 

 into modifications of three important factors : — 



(a) The gonads or germ-glands, in which are developed the 



^ Hermaphroditism seems to occur in («) whole families, e.g. Anatinidae and the 

 SqUibrancliia ; (h) genera, e.g. Cyclas, Pisidium ; (c) single species, e.g. in the 

 generally dioecious genera Ostrea, Pecten, Cardium. 



"^ dvu, two ; /xovos, single ; ydvos, semen ; irdpos, passage. 



