SUPPLEMENTARY NOTES 
006 
has also ascertained the existence of small red eye-specks at the 
extremity of the rays of the Asterias. 
P. 328. The speci^c gravity of a body, is its weight, com- 
pared with the weight of another body, whose magnitude is the 
same; hence, if a body which occupies any given space in water 
be contracted into a smaller magnitude, whilst its absolute weight 
remains the same, it becomes specifically heavier. Supposing 
the absolute weight of the body of the Nautilus, and also that of 
its pericardial fluid, to be the same as that of an equal bulk of 
water, the body, when immersed, would always displace a 
quantity of water, equal to its bulk. The presence of the peri- 
cardial fluid within the body, (i. e. within the Pericardium), or 
its removal from it into the shell, would not affect the spec’ific 
gravity of the body, because the magnitude of the body varies 
according as the pericardium is either empty, or distended with 
its peculiar fluid. But, as the magnitude of the shell is con- 
stantly the same, whilst the quantity of matter within it varies, 
as the pericardial fluid enters or leaves the siphuncle, its specific 
gravity is varied accordingly, being increased, when the fluid 
enters the siphuncle (compressing the air within the air-cham- 
bers), and diminished, when this fluid returns from the siphuncle 
into the body. 
When the animal, preparing to rise, emerges from its shell, and 
the pericardial fluid, returning from the siphuncle into the peri- 
cardial sac, enlarges the body by the distension of this sac, the 
absolute weight of the body and shell together remains the same 
but the specific gravity of the whole is diminished by this increase 
of the bulk of the body, and the animal floats. When preparing 
to sink, it shrinks back into its shell, and compressing the peri- 
cardial sac, forces its contents into the siphuncle, the bulk of 
the body IS diminished by the collapse of this sac to an amount 
equal to the difference between the bulk of the distended and 
contracted sac, the whole becomes specifically heavier, and the 
animal sinks. 
For the sake of simplifying the problem we have supposed 
the specific gravities both of the pericardial fluid, and of the 
body of the animal, to be the same as that of water. If, as Mr. 
