CHAPTER VI 
REPRODUCTION 
HE 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 Littovina, 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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