SUPPLEMENTARY NOTES. 607 
Owen affirms, the pericardial fluid is more dense than water^ 
its transfer into the siphuncle will be more efficacious in causing 
the shell to sink, because a fluid, whose specific gravity is 
greater than that of an equal bulk of water^ is added to the 
shell, without increasing its magnitude ; but when the same 
fluid returns into the body, the consequent addition to the 
specific gravity of the body, is only the difference between the 
specific gravity of this fluid and that of water; and this is more 
than counterbalanced by the dwiinution of sjjecijic gravity 
which the body undergoes from the expansion of the retractile 
tentacula, and consequent enlargement of their magnitude. 
The same tentacula, when the animal shrinks back into its shelly 
are contracted into a smaller magnitude, and increase the ten- 
dency of the shell to sink. 
In the Water balloon and apparatus connected with it, re- 
ferred to at p. 318 and p. 327, the tall glass, and membrane 
wliich covers it, represent the Pericardium of the Nautilus ; the 
water which fills the glass acts like the pericardial fluid, and if 
a small empty bladder were attached to the neck of the Balloon^ 
and suspended, like an artificial siphuncle, within its cavity, the 
bladder, when filled with water, would represent the siphuncle 
of the Nautilus, when filled with pericardial fluid ; and the air 
within the chamber of the Balloon, would represent that withiri 
the chambers of the Nautilus. 
The difference would be, that in the case of the Nautilus, the 
entire Pericardium is a flexible membrane, and that nearly the 
whole of the pericardial fluid may be forced into the siphuncle ;. 
whilst in the water Balloon, the membrane only at the top of the 
glass is flexible, and a small part only of the water in the glass- 
can be forced into the Balloon. 
The principle which causes a change in the specific gravity^ 
by varying the quantity of matter, within the shell and within, 
the Balloon^ without varying their respective magnitudes, is the 
same. 
P. 32y *. The Tentacula which when expanded around the 
head, would impede any jjrogressive motion of the animal, would 
follow the retrograde body and shell, without causing any 
