REPRODUCTION AND SEX 



497 



different from the male. She is much larger, she has two "arms" 

 peculiarly modified to secrete a unique shell, not homologous with 

 other Cephalopod shells, which is chiefly used as a brood-chamber for 

 the developing ova. 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. 



These are familiar facts, but we do not know of any evidence for 

 supposing that the immediate ancestors of the Paper-nautilus had 

 an external shell or modified arms such as the female now shows. 

 There is no hint of such a thing. Moreover, the shell is not for living 



Fig. 



71 



Paper-nautilus (Argonauta), the female with the brood-chamber shell (SH), 

 and in proportion the pigmy male (M) with a "hectocotylised" arm (H), 

 laden with sperms. A, one of the "arms" that secretes the delicate shell; 

 F, the locomotor funnel through which water is forcibly expelled. One 

 of the eyes is shown. Most books show the shell upside down. 



in, but for the protection of the eggs, it is a cradle not a house, 

 and it has no meaning except in the female. 



Again, let us take one of those very interesting cases where the 

 female has something definite and positive which the male has not — 

 the frog Nototrema with its dorsal pouch in which the eggs are 

 carried. Is there any warrant for supposing that this was once a 

 specific character? 



Another case in point may be found in the so-called claspers of 

 male Selachians and Chimaeroids. In a fish like the skate they are 

 very conspicuous sex-characters; they are highly speciahsed struc- 



VOL. I KK 



