ITS ACTION AT THE SURFACE. 329 
is best adapted to the retrograde motion, which 
a Sepia derives from the violent ejection of 
water through its funnel (k) ; * thus far, the air 
chambers serve to maintain both the shell and 
body of the animal in a state of equilibrium at 
the surface. 
The mode of operation of the siphuncle and air 
chambers, in the act of sinking suddenly from 
the surface to the bottom is explained in the note 
subjoined, t 
* See Sup. Note. 
t It appears from the figure of the animal, PI. 34, with which 
I have been favoured by Mr. Owen, that the upper extremity 
of the siphuncle marked by the insertion of the probe b., termi- 
nates in the cavity of the pericardium p, p. As this cavity con- 
tains a fluid, excreted by the glandular follicles d, d., and is 
apparently of such a size that its contents would suffice to fill 
the siphuncle, it is probable that this fluid forms the circulating 
medium of adjustment, and regulates the ascent or descent of 
the animal by its interchange of place from the pericardium to 
the siphuncle. 
When the arms and body are expanded, the fluid remains in 
the pericardium, and the siphuncle is empty, and collapsed, and 
surrounded by the portions of air that are permanently confined 
within each air chamber ; in this state, the specific gravity of the 
body and shell together is such as to cause the animal to rise, 
and be sustained floating at the surface. 
When, on any alarm, the arms and body are contracted, and 
withdrawn into the shell, the retraction of these parts, causing 
pressure on the pericardium, forces its fluid contents into the si- 
phuncle ; and as the quantity of matter within the shell is thus 
increased, without increasing its magnitude, whilst the specific 
gravity of the body remains unaltered by the removal of this fluid 
from the pericardium, accompanied by a simultaneous diminution 
of the magnitude of the body, the specific gravity of the entire 
animal is increased, and it begins to sink. 
