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SEXUAL DIMORPHISM 



the sexes, but the only conspicuous external difference is the 

 large penis of the males, which is situated in the mantle- 

 cavity, and is not retractile. In the Heteropoda, however, 

 which are free-swimming Prosobranchs with small shells, a 

 typical secondary sexual character occurs. In Pterotrachea 

 there is, on the foot of the male only, a sucker which is 

 evidently used for attachment to the female. In other genera, 

 Atlanta and Carinaria, the sucker occurs in both sexes. 

 Here there is no difficulty in understanding how the special 

 muscular action of the foot could give rise to a special 

 sucker, and as in other cases the development in both sexes 

 in certain cases probably results from the fact that both 

 perform the corresponding muscular contractions. 



The Lamellibranchiata are, with few exceptions, such as the 

 scallop and oyster, of separate sexes, but as the fertilisation is 

 automatic, secondary sexual differences are scarcely at all 

 developed. The only instances are in those species in which 

 the female nurses the developing ova within the mantle- 

 cavity, in the folds of the gill-lamellae. In these species, for 

 example the fresh-water Unionidse, the shell of the female is 

 more convex, a fact which can only be regarded as the result 

 of the distension caused by the ova and embryos. The shell 

 once formed, is, it is true, not likely to be altered in shape by 

 pressure from within the mantle-cavity, but when the mantle- 

 cavity is distended the edges of the shell will be separated 

 slightly, and the edges of the mantle meeting to cover the gap 

 will secrete additions to the edge of the shell in a steeper 

 curvature than in the case of the male. 



In the Cephalopoda the males are, at least when mature, 

 conspicuously distinguished from the females by the modifica- 

 tion of one or two of the arms, which are said to be hecto- 

 cotylised. The name hectocotylus was originally given to an 

 elongated cylindrical body, provided with numerous suckers, 

 which was found in the mantle-cavity of the female Argonauta, 



