210 INVERTEBRATE ANIMALS. 
arms of the female. These arms are bent backwards, so as to 
allow the animal to live in the shell; but there is no organic 
connection between the shell and the body of the animal. The 
shell of the Pearly Nautilus, on the other hand, is secreted by 
the mantle, and is organically connected to the animal. It is 
coiled into a spiral (fig. 113), but it differs from the shell of the 
Argonaut in being divided into a series of chambers by means 
of shelly partitions, which are connected together by a tube 
or “siphuncle,” the animal itself living in the last and largest 
chamber only of the shell. 
The Cephalopoda are divided into two extremely distinct and 
natural orders, termed respectively Dibranchiata and Tetra- 
branchiata, according as they have ¢wo or four gills or branchiz. 
The Débranchiata comprise the Cuttle-fishes, Squids, Cala- 
maries, and Paper Nautilus, and they are characterised by 
Big. 112.—Argonauta argo, the Paper Nautilus, female. The animal is repre- 
sented in its shell, but the webbed dorsal arms are separated from the shell 
which they secrete, and which they ordinarily embrace. 
being almost invariably destitute of any external shell; by 
never having more than eight or ten arms, which are always 
