SUPPLEMENTARY NOTES. 455 



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. 

 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 coun- 

 terbalanced by the diminution of specific 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 shell, are con- 

 tracted into a smaller magnitude, and increase the tendency of the 

 shell to sink. 



In the Water balloon and apparatus connected with it, referred 

 to at p. 241 and p. 248, the tall glass, and membrane which 

 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 sus- 

 pended, 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 within the chambers 

 of the Nautilus. 



The diflTerence 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; 



