CHAPTER VI 

 REPRODUCTION 



THE sexes in the majority of the molluscs 

 are separate. Hermaphroditism accompanies 

 specialization, and only obtains normally in the 

 Neomeniina among the Amphineura, the Euthy- 

 neura and some genera of Streptoneura among the 

 Gastropoda, the Anatinacea, and a few other 

 isolated instances among the Pelecypoda. 



In those forms in which the sexes are separate 

 there is often a definite sexual dimorphism. Fre- 

 quently the female is the larger and the more tumid, 

 as in Littorina, Vivipara, and the British land oper- 

 culate, Pomatias elegans; while in Lacuna pallidula the 

 female outweighs the male by ten to one. Differ- 

 ences are also exhibited in the mouth of the shell 

 in Littorina obtusata, in the operculum in some 

 species of Cerithium, in the teeth of the radula in 

 Nassa, while in Vivipara the right tentacle of the 

 male is curiously modified. 



In the Pelecypoda dimorphism is recognizable in 

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