floating at the surface, with the upper portion of the shell raised above the water, and kept in a vertical position by means of the included air (see Pl. 31. Fig. 1.); this position 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.
Secondly, The next point to be considered is the mode of operation of the siphuncle and air-chambers, in the act of sinking suddenly from the surface to the bottom. These are explained in the note subjoined.[1]
- ↑
It appears from the figure of the animal, Pl. 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., terminates in the cavity of the pericardium p, p. As this cavity may contain a fluid, more dense than water, 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 drawn into the shell, the retraction of these parts, causing pressure on the exterior of the pericardium, forces its fluid contents downwards into the siphuncle; and the bulk of the body being thus diminished, without increasing the bulk of the shell, into whose cavities the fluid is withdrawn, the specific gravity of the whole mass is suddenly increased, and the animal begins to sink.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 pericardia fluid; and returning, by its elasticity, to its former state, as soon as the pressure of the arms and body is withdrawn from the pericardium, forces the fluid back again into the cavity of this organ; and thus the shell, diminished as to its specific gravity, has a tendency to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 pericardia fluid, therefore, will be always in the peri-
