MOLLUSCA 289 



the well-known Paper Nautilus. This genus belongs to the 

 order Octopoda. It was afterwards discovered that the hecto- 

 cotylus was a detached arm of the male Argonauta, and that 

 its function was to convey the spermatophores into the mantle- 

 cavity of the female. It is now known that the modified arm 

 of the male is the third of the left side, and that it develops 

 in a peculiar manner. In the place of an ordinary arm or 

 tentacle, where the third left arm ought to be, there is found 

 a pear-shaped membranous capsule, and in the interior of 

 this is found the arm or tentacle proper, rolled up in a coil. 

 At the time of sexual maturity the capsule bursts and the 

 tentacle is released, still, however, attached at its base. The 

 walls of the capsule are everted and form a receptacle at the 

 back of the arm, into which the spermatophores are received, 

 and when the arm charged with the spermatophores is inserted 

 into the pallial cavity of the female, it becomes detached 

 and is left in that cavity, where it retains its vitality and 

 power of motion for some time. 



It is evident from this that the so-called capsule in which 

 the arm develops is formed merely by the concrescence of 

 membranous outgrowths of the modified arm, with the surface 

 of the body around the base of the arm. Since the walls of 

 the capsule after rupture form the receptacle of the spermato- 

 phores, it is probable that the peculiar mode of development 

 is due merely to the precocious development of the mem- 

 branous sides of this receptacle. Granted such a precocious 

 development in an arm which originally remained coiled up 

 until the time for its special function arrived, the membrane 

 may have been extended by muscular contraction over the 

 coiled arm with its edges in apposition, until in the course of 

 evolution concrescence took place. 



Supposing the effects of external stimulation do not become 

 hereditary, we have to form a conception of the evolution of 

 this hectocotylised arm by the process of indefinite variation 



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