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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 /Zectocotylus was given 
it. The male, after losing 
its arm, always reproduces 
it again. In the female 
Argonauta the eggs are 
earried about in the shell; 
this is the only member of 
the Octopoda which has a 
shell, and it does not cor- 
respond with the shell 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 
Fic. 130.—Argonauta argo, the Paper shell is partially sur- 
Nautilus, female. The animal is repre- 
sented in its shell, but the webbed dorsal rounded by folds of the 
arms are separated from the shell which mantle, and in other forms 
they ordinarily embrace. ie aitalciceshade riser fae 
gether so that the shell comes to le 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 siphunele, 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 
