ITS ACTION AT THE SURFACE. 249 



First, The animal was seen and captured by Mr. Bennett, 

 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 PI. 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 sinJdrig suddenly from the surface to the bottom. These 

 are explained in the note subjoined.! 



* See Sup. Note. 



t It appears from the figure of the animal, PI. 34, with whicii I have 

 been favoured by Mr. Owen, that the tjpper extremity of the siphuncle 

 marked by tiie insertion of the probe b., terminates in tiie cavity of the peri- 

 cardium p, p. As this cavity contains a fluid, more dense tiian 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 pericar- 

 dium, 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 

 cauise 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When, on any alarm, the arms and body are contracted, and withdrawn 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 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 witlidrawn, 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 ^he 

 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, di.ninished as to its specfic gravity, 

 has a tendency to rise. 



