88 THE LIFE OF THE MOLLUSCA 
Unio 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 Vivipava (Plate II., Fig. 7) 
among freshwater Gastropods, Cymba and many 
species of Littovina among marine; several isolated 
instances also occur among the land Pulmonates— 
e.g., Pyramidula, Opeas, some Pupillide, etc. In the 
