Thirdly, It remains to consider the effect of
the air, supposing it to be retained continually
within the chambers, at the bottom of the sea.
Here, if the position of the moving animal be
beneath the mouth of the shell, like that of a
snail as it crawls along the ground, the air within
the chambers would maintain the shell, buoyant,
and floating over the body of the animal in a
thus diminished, without increasing the bulk of the shell, into
whose cavities the fluid is withdrawn, the specific gravity of the
whole mass is suddenly increased, and the animal begins to sink.
The air within each chamber remains under compression, as
long as the siphuncle continues distended by the pericardial
fluid; and returning, by its elasticity, to its former state, as soon
as the pressure of the arms and body is withdrawn from the
pericardium, forces the fluid back again into the cavity of this
organ ; and thus the shell, diminished as to its specific gravity,
has a tendency to rise.
The place of the pericardial fluid, therefore, will be always in
the pericardium, excepting when it is forced into and retained in
the siphuncle, by muscular pressure, during the contraction of
the arms and body closed up within the shell. When these are
expanded, either on the surface, or at the bottom of the sea,
the water will have free access to the branchiae, and the movements
of the heart will proceed freely in the distended pericardium;
which will be emptied of its fluid at those times only, when
the body is closed, and the access of water to the branchiae consequently
impeded.
The following experiments shew that the weight of fluid requisite
to be added to the shell of a Nautilus, in order to make it
sink, is about half an ounce.
I took two perfect shells of a Nautilus Pompilius, each weighing
about six ounces and a half in air, and measuring about seven
inches across their largest diameter; and having stopped the
Siphuncle with wax, I found that each shell, when placed in fresh
water, required the weight of a few grains more than an ounce to
vertical position, with little or no muscular exertion,
and leave the creature at ease to regulate
the movements of its tentacula (p) in crawling
and seizing its prey.*
Dr. Hook considered (Hook’s Experiments,
8vo. 1726, page 308) that the air chambers
were filled alternately with air or water; and
Parkinson (Organic Remains, vol. iii. p. 102),
admitting that these chambers were not accessible
to water, thinks that the act of rising
or sinking depends on the alternate introduction
o f air or water into the siphuncle; but he is at
a loss to find the source from which this air
could be obtained at the bottom of the sea, or to
make it sink. As the shell, when attached to the living animal,
was probably a quarter of an ounce heavier than these dry dead
shells, and the specific gravity of the body of the animal may
have exceeded that of water to the amount of another quarter of
an ounce, there remains about half an ounce, for the weight of
fluid which beings introduced into the siphuncle, would cause
the shell to sink; and this quantity seems well proportioned
to the capacity both of the pericardium, and of the distended
siphuncle.
* If the chambers were filled with water, the shell could not
be thus suspended without muscular exertion, and instead of
being poised vertically over the body, in a position of ease and
safety, would be continually tending to fall flat upon its side;
thus exposing itself to injury by friction, and the animal to
attacks from its enemies. Rumphius states, that at the bottom,
He creeps with his boat above him, and with his head and
barbs (tentacula) on the ground, making a tolerably quick progress.
I have observed that a similar vertical position is maintained
by the shell of the Planorbis corneus, whilst in the act of
crawling at the bottom.