MOLLUSCA 291 



of the peculiar development. It may be asked, why should 

 precocious development occur ? To which the reply is that 

 numerous facts point to the induction that generally when the 

 growth of a special part is greatly stimulated by special use 

 or irritation, the development of that part is in the course of 

 generations accelerated. The detachment of the arm, which 

 is now a normal occurrence, would of course be attributed to 

 the mechanical separation of it occasionally occurring in the 

 process of copulation. 



In Argonauta the peculiarity of the copulatory tentacle is 

 by no means the only difference between male and female. 

 The male is smaller than the female. The latter has a shell, 

 while the former has none. This delicate, beautiful shell, 

 which gives rise to the name Paper Nautilus, is formed, not 

 by the surface of the body or mantle, but by the inner surface 

 of two modified arms. In this case it is the two most dorsal 

 arms which are specialised. Each of these is enlarged into a 

 flat membranous expansion, and one is applied to each side of 

 the body. On the inner sides of these expansions is secreted 

 the thin, delicate shell which encloses the body. The eggs of 

 the female are attached to the shell. It is possible that the 

 support of the eggs was the original purpose for which the 

 dorsal arms were turned back and applied to the sides of the 

 body. In any case it is difficult to conceive of the modifica- 

 tion of these arms taking place apart from the special mode 

 in which they are held by the animal. In this instance we 

 see the usual association of specific and hereditary modification 

 with a special use of the organ, confined to one sex. With 

 regard to the secretion of the shell the difficulty is greater. 

 At present I am unable to attribute the secretion of the shell 

 to particular conditions or stimulations involved in the 

 attitude of the arms. It is known that the suckers of the 

 cephalopod arm are often coated with a horny secretion, and 

 the shell may perhaps represent the connected and calcified 



