90 SEX 



logous with other cephalopod shells, which 

 is used as a brood -chamber for the developing 

 ova (see Fig. 17). The small male has no 

 such shell and no such modification of two 

 of the arms. When he is sexually mature, one 

 of his arms becomes laden with sperm-packets 

 and is discharged as a " hectocotylus " into 

 the mantle cavity of the female (see Fig. 18). 

 These are familiar facts, but we do not 



FIG. 18. The minute male of the Paper Nautilus Argonauta 

 argo showing the mouth surrounded by eight arms, 

 one of which has been " hectocotylised," i. t. much 

 specialised for reproduction. The locomotor funnel and 

 the mantle opening may be seen to the left, one of the 

 eyes to the right. Twice natural size. (After Miiller.) 



know of any evidence for supposing that the 

 ancestors of the argonaut ever had an ex- 

 ternal shell of the argonaut type or modified 

 arms such as the female now shows. There 

 is no hint of such a thing. Moreover, the 

 shell is not for living in, but for the protection 

 of the eggs ; it is a cradle, not a house, and it 



