88 THE LIFE OF THE MOLLUSCA 



Unto batavus, U. tumidus, and Lampsilis, in which the 

 female is rather broader than the male, and in 

 Astarte, in which the border of the shell is smooth 

 in the male, but crenulated in the female. 



Among the Cephalopoda sexual dimorphism is 

 very marked, especially in the Argonaut, the female 

 of which may attain to fifteen times the length of 

 the male. Generally the males are more slender 

 than the females, but in Nautilus the hood of the 

 animal and the mouth of the shell in the male are 

 wider than in the female. The male Cuttlefish is 

 also remarkable for the curious modification of one 

 of its arms, known as " hectocotylization " (Plate 

 XXV., Fig. 14). At the time of pairing this arm 

 becomes charged with the spermatophores, and is 

 the vehicle for their transference to the female. In 

 some cases — as, for example, the Argonaut — the hecto- 

 cotylus becomes detached from the male and is able 

 to live and move about for a considerable time. It 

 has even been described as a parasite on the female. 



Most of the Mollusca are oviparous — that is to 

 say, lay eggs ; in a few cases the eggs hatch within 

 the body of the parent, and the young are brought 

 forth alive, as in Calistochiton among the Amphi- 

 neura, Melania and Vivipam (Plate II., Fig. 7) 

 among freshwater Gastropods, Cymba and many 

 species of Littorina among marine ; several isolated 

 instances also occur among the land Pulmonates — 

 e.g., Pyramidula, Opeas, some Pupillidse, etc. In the 



