CEPHALOPODA. 429 
innermost layer of the shell. This is exposed after the soft 
organic stratum and the median porcellanous layer which 
bears bands of colour have been worn away, or dissolved 
in a dolphin’s stomach, or artificially treated with acid. 
The beautiful shell is a spiral in one plane, divided into 
a set of chambers, in the last of which the animal lives, 
while the others contain gas. The young creature inhabits 
a tiny shell curved like a horn; it grows too big for this, 
and proceeds to enlarge its dwelling, meanwhile drawing 
itself forward from the older part, and forming a door of 
lime behind it. This process is repeated again and again; - 
as an addition is made in front, 
the animal draws itself forward 
a little, and shuts off a part of 
the chamber in which it has 
been living. All the compart- 
ments are in communication by 
a median tube of skin—the 
siphuncle—which is in part cal- 
careous. , 
It has been suggested ‘that 
“each septum shutting off an 
air-containing chamber is formed 
during a period of quiescence, 
probably after the reproductive 
act, when the visceral mass’ of 
the Nautilus may be slightly 
shrunk, and gas is secreted from Z Sree Satie may 
the dorsal integument so as to “Fe Ee ee eld. 
fill up the space previously 
occupied by the animal.” 
There can be no confusion between the beautiful shell of 
the cuttlefish called the paper Nautilus (Avgonauta argo) 
and that of our type. For it is only the female Argonaut 
which bears a shell; it is not chambered, and is a shelter 
for the eggs—a cradle, not a house. It seems to be 
formed by two of the arms. 
It is instructive to compare the Nautilus shell with that 
of some Gasteropods, for there also chambers are occasion- 
ally, formed. But these arise from secondary.alterations of 
an originally continuous spiral. The Gasteropod shell is 
