248 NAUTILUS POMPILIUS. 



to the siphuncle, (n.,) we shall find in these organs an hy^* 

 draulic apparatus for varying the specific gravity of the 

 shell; so that it sinks when the pericardial fluid is forced into 

 the siphuncle, and becomes buoyant, whenever this fluid 

 returns to the pericardium. On this hypothesis also the 

 chambers would be permanently filled with air alone, the 

 elasticity of which would readily admit of the alternate 

 expansion and contraction of the siphuncle, in the act of 

 admitting or rejecting the pericardial fluid. 



The principle to which we thus refer the rising and sink- 

 ing of the hving Nautilus, has been already stated (P. 241) 

 to be the same which regulates the ascent and descent of 

 the Water Balloon: the application of external pressure 

 through a membrane that covers the column of water in 

 a tall glass, forces a portion of this water into the cavity, 

 or 'single air-chamber of the balloon, which immediately 

 begins to sink ; on the removal of this pressure, the elasti- 

 city of the compressed air causing it to return to its former 

 volume, again expels the water, and the balloon begins to 

 rise.* 



I shall conclude this attempt to illustrate the structure and 

 economy of fossil Nautili by those of the living species, by 

 showing in what manner the chambers of the pearly Nau- 

 tilus, supposing them to be permanently filled only with air, 

 and the action of the siphuncle,f supposing it to be the recep- 

 tacle only of a fluid secretion, interchanging its place alter- 

 nately from the siphuncle to the pericardium, J would be sub- 

 sidiary to the movements of the animal, both at the surface, 

 and bottom of the sea. 



* See Sup. Note. 



t Tlie substance of the siphuncle is a thin and strong membrane, with no 

 appearance of muscular fibres, by whicli it could contract or expand itself; 

 its functions, therefore, must have been entirely passiue, in the process of 

 admitting' or ejecting any fluid to or from its interior. — (See Owen's Me- 

 moir, p. 10.) In our first edition it was stated erroneously that the siphun- 

 cle had no appearance of muscular fibres. 



t See Snp. Note. 



