ZOOLOGY 



certain Octopods this modification is carried much further. In 

 Argonauta argo the third arm on the left, and in Ocythoe tuber- 

 culata the same arm on the right, becomes detached from the 

 male, and is placed in the mantle cavity of the female. It carries 

 a small sac charged with spermatophores, and was at one time 

 looked upon as a parasite, and the name Hectocotyliis was given 



it. The male, after losing 

 its arm, always reproduces 

 it again. In the female 

 Argonauta the eggs are 

 carried about in the shell; 

 this is the only member of 

 the Octopoda which has a 

 shell, and it does not cor- 

 respond with the sheU of 

 other Cephalopods, but is 

 formed from the expanded 

 ends of the two dorsal 

 arms. 



In other Dibranchiata 

 the shell varies from the 

 external coiled chambered 

 shell of Spirula to the 

 horny pen of Loligo. Even 

 in Spirula (Fig. 125) the 



Fig. ISO. — Argonauta argo, the Paper shell is partially Sur- 



Nautilus, female. The animal is repre- _„ -i„j i ,, p„i j„ „j; 4.-u„ 



sented in its shell, but the webbed dorsal funded by _ folds of the 



arms are separated from the shell which mantle, and in Other formS 



they ordinarily embrace. ^^^ ^^^^^ ^^^^ ^^^^^ ^_ 



gether so that the shell comes to lie in a closed sac. In the 

 Tetrabranchiata, Nautilus and the extinct Ammonitidae, the 

 shell is external, and chambered. The animal lies in the 

 last-formed chamber, and closely fits it. The chambers are 

 separated from one another by septa, and the whole is traversed 

 by a membranous tube, the siphuncle, which is a continuation 

 of the integument of the animal. The chambers are full of 

 a gas probably secreted by the dorsal integument, and they 

 doubtless serve as a float. 



In Nautilus the fore-foot is broken up into certain flattened 



