318 MECHANISM OF NAUTILUS POMPILIUS. 
till it terminates in the smallest chamber at the 
inner extremity of the shell. I shall presently 
attempt to show how by means of a peculiar fluid, 
admitted into or abstracted from this pipe, the 
animal has the power to increase or diminish its 
specific gravity, and to sink or float accordingly ; 
as the floating portion of that beautiful toy the 
Water balloon is made to descend or ascend by 
means of water forced into, or abstracted from 
its interior. (See P. 327.) 
The motion of the Nautilus, when swimming, 
with its arms extended, is retrograde, like that of 
the naked Cuttle Fish, being produced by the 
reaction of water, violently ejected from the 
funnel (k). 
The position assumed during this operation is 
that which is best adapted to facilitate its pas- 
sage through the water, as it places foremost that 
part of the shell, which approaches most nearly 
in form to the prow of a boat. The fingers and ten- 
tacula (p,p), are here represented as closed around 
the beak, which is consequently invisible ; when 
the animal 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 were found in 
the stomach of the individual here represented. 
