of the ActioTt of the Siphuncle in the Pearly Nautilus. 507 



animal to rise, and be sustained floating at the surface*." 

 If this explanation be correct, the nautilus cannot remain 

 at the bottom unless the siphon is distended by the 

 retreat of the animal into the last chamber of its shell. 

 What a prodigious muscular effort must, therefore, be 

 constantly required to keep the nautilus at the bed of 

 tlie sea ! Again, it may be asked, how is the nautihis to 

 seize its prey at the bottom, seeing the instant its head is 

 protruded to search, and its arms expanded to seize it, 

 " the fluid would be forced back again into the cavity 

 of the pericardium, and thus the shell, diminished as to 

 its specific gravity, would have a tendency to risef"? 

 This theory, therefore, is at variance with the inference 

 obtained from an examination of the organization of the 

 nautilus, that it seeks its prey at the bottom, and is but 

 an occasional visitor at the surface : it is opposed like- 

 wise to a well-known law of the animal oeconomy, that 

 mechanical contrivances are always substituted, where 

 long-continued action is required, to ceconomise the ex- 

 penditure of muscular power; a familiar example is af- 

 forded in the Conchifera^ where an elastic ligament is em- 

 ployed to keep open the valvesof the shell, adductor mus- 

 cles being furnished for the occasional closing of the same. 

 If we test the theory by this principle, we find that to 

 keep the tube distended and the air compressed, in order 

 that the nautilus may remain at the bottom, a constant 

 muscular eflbrt would require to be sustained, in order 

 to overcome the elasticity of the confined air, the expan- 

 sion of which is, according to Dr. Buckland, the power 

 by which the siphuncle is emptied of its aqueous con- 

 tents. The explanation which 1 have ventured to pro- 

 pose is in perfect harmony with the oeconomical law 

 alluded to; for whilst the nautilus is at the bed of the 

 sea, the muscular powers of the pericardium would be 

 in a passive state, just as the adductor muscle of the 

 conchifer is in a state of repose when the valves are kept 

 open by the elastic hinge ; no effort 'would be required 

 to keep the animal in its natural situation ; as in the 

 Conchifera^ so it may be in the nautilus ; an effort of 

 the will shuts the valvesof the former, and the contrac- 

 tion of the pericardium, by ejecting the watery ballast 

 from the siphuncle of the latter, would allow it to change 

 its feeding ground, ascend to a higher stratum of the 

 water, or to its surface if required. 



* Bridgcwater Treatise, vol. i. p. 329 note. f Ibid., p. 330. 



