SEXUAL DIMORPHISM. 



741 



10 were male and 16 female. The actual distribution of the numbers for N. pompilius 

 is given in the subjoined table. 



In contrast with the race of the Argonauts or Paper Nautili where the male is 

 strikingly smaller than the female, which alone carries a shell of peculiar construction, 

 the males of the Pearly Nautilu.i are rather larger than the females. This predominance 

 of the male is due to its possession of a remarkable organ, the spadix, placed beside 

 the mouth within the outer whorl of tentacles and formed, as first described by the 

 Dutch anatomist Van der Hoeven, by the combination of four modified tentacles belonging 

 to the inner whorl (see below). The presence of the bulky sjjadi.x in the midst of the 

 cephalic comple.x which, in the retracted condition, is concealed below the hood, determines 

 an increase in the dimensions of the latter, which must obviously find expression in the 

 size of the orifice of the shell. 



97—2 



