318 MECHANISM OF NAUTILUS POMPILIUS. 
ing tube (y) in each successive transverse plate, 
till it terminates in the smallest chamber at the 
inner extremity of the shell. By means of this 
pipe, the animal has power to increase or dimin- 
ish its specific gravity, as Fishes do, by disten- 
ding their membranous air bladder with air, 
or by causing it to collapse. When the pipe 
of the Nautilus is filled with any fluid, the 
weight of that fluid, being added to the body and 
shell, renders the mass specifically heavier than 
water, and the animal sinks. When it is inclined 
to rise, it withdraws the fluid from the pipe, 
and thus again, becoming specifically lighter, 
rises upwards to the surface. 
The motion of the Nautilus, when floating, 
with its arms expanded, is retrograde, like that 
of the naked Cuttle Fish, being produced by the 
reaction of water, violently ejected from the 
funnel (k). The fingers and tentacula (p, p,) 
are here represented as closed around the beak, 
which is consequently invisible ; when the ani- 
mal is in action, they are probably spread forth 
like the expanded rays of the sea Anemone. 
The horny beak of this recent Nautilus (See 
PI. 31, Fig. 2, 3) resembles the bill of a Parrot. 
Each mandible is armed in front, with a hard 
and indented calcareous point, adapted to the 
office of crushing shells and crustaceous animals, 
of which latter, many fragments w'ere found in 
the stomach of the individual here represented. 
