SHELL OF THE PEARLY NAUTILUS. 103 



able in its appearance, and might be mistaken for the domicile 

 of an overgrown snail, only that its sides are considerably flat- 

 tened, and that it is somewhat too artistically finished off at the 

 mouth. Cut into halves, however, and exposed thus in section, 

 it is seen to be divided into a series of chambers, the outermost of 

 which is large and roomy, and contains the body of the Nautilus, 

 whilst the others are occupied only by air, and gradually decrease 

 in size as they approach the inner extremity of the shell. The 

 partitions between the chambers are firm plates of shelly matter, 

 which, as they are continued from the side walls, make every 

 compartment perfectly air-tight, and thus convert the shell into 

 a float, which buoys up the Nautilus in the water. A communi- 

 cation is kept up between the several chambers by means of a 

 membranous tube, termed the sipliunde, which, originating in 

 the body of the Nautilus in the outer chamber, passes through 

 all the partitions in succession, and has been supposed, though 

 apparently without much reason, to be the principal agent 

 actively concerned in that alteration of the specific gravity of 

 the Nautilus, by which it is enabled at will to ascend to the 

 surface from the sea-bottom, or instantly sink again if danger 

 threaten. 



This ascending and descending in the water is an interesting 

 point in the economy of the Nautilus, and one which has abun- 

 dantly shown that philosophers are not exempt from the common 

 failing of preferring novel and fanciful methods of accounting for 

 unusually curious phenomena, even when familiar and common- 

 place explanations would equally answer the purpose. Thus the 

 most ingenious theories have been propounded to explain the 

 modus operandi of these visits of the Nautilus to the sunlight 

 and back, the siphuncle being in all of them alike credited with 

 the chief share in the transaction. 



But there is good reason to believe it has little or nothing to 

 do with the matter, and that the change of place is effected by a 

 simple contrivance, which, any day in summer, may be seen illus- 

 trated by another little Mollusc common in almost every roadside 

 ditch in the country. The fact is, that, as Professor Owen has satis- 

 factorily shown, the Nautilus, contracted within its buoyant shell, 

 is so little heavier than its own bulk of water, that by protrud- 

 ing its body beyond the aperture of the shell, and spreading 

 itself abroad so as to displace a greater volume of the fluid, it 



