134 TYPES OF SEXUAL DIFFERENCE chap. 



2. The sexes are united in 



Grasteropoda Opistliohranchiata. 

 G-asteropoda Pulmonata. 

 Certain Pelecypoda} 



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

 no means universal. In some instances, — e.g. Vermetus^ Magi- 

 lus, Patella^ ffaliotis, Crepidula, Chiton^ the Sccqohopoda — the 

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

 (many TroeJius') 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 markiedly the case in Littorina, Biic- 

 cirium^ and all the Cephalopoda ; in Argonauta the difference is 

 extreme, the male not being more than 1 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 Opis- 

 thobranchiata, including all the Pteropoda ; to the latter QMono- 

 gonopora ^) 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 them- 

 selves into modifications of three important factors: — 



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



1 Hermaphroditism seems to occur in (a) whole families, e.g. Anatinidae 

 and the Septibranchia ; {b) genera, e.g. Cyclas, Pisidhnn ; (c) single species, 

 e.g. in the generally dioecious genera Ostrea, Pecten, Cardium. 



2 Suw, two ; fibvos., single ; 761'os, semen ; tto/jos, passage. 



