47 



sumes that the seat of the accumulation of the gaseous fluid is the membranous 

 tube, and considers it to have a corresponding power of dilatation and con- 

 traction. Those who incline towards this opinion may derive from the preceding 

 account circumstances in some measure explanatory both of the source of the 

 supposed contained gas, and of the way by which it afterwards escapes : the one 

 may be considered as a secretion of the artery which is continued down the 

 membranous tube ; and the comnmnication which exists between the tube and 

 branchial cavity through the medium of the pericardium, may be deemed ex- 

 planatory of the other part of the process. 



But it must be admitted, on the other hand, that the size of the artery seems 

 barely adequate to support the vitality of the membrane, much less to effect a 

 secretion, for which in Fish, (at least such as have an outlet to their air-bladders,) 

 an ample gland appears to be indispensable ; and with respect to the outlet, the 

 oblique and contracted nature of the passage is ill calculated to allow of an 

 escape of the gas sufficiently rapid to answer as a self-preserving action, or a 

 means of defence against sudden assaults. 



Much, indeed, remains to be done before the theory of the chambers and 

 siphuncle can i-est on the sound basis of experiment and observation. Mr. Ben- 

 nett's observation, — that the contents of the deserted chambers in the living 

 animal are liquid, — is an important addition to its history ; though it may still 

 be doubted whether their contents are the same under all circumstances, even 

 during the lifetime of the animal ; and the nature of the fluid, its proportional 

 quantity*, and the precise disposition and contents of the membranous tube, 

 still remain to be determined. 



From the adhesion of the entire circumference of the mantle to the shell by 

 means of the horny girdle, I am, on the contrary, inclined to suppose that the 

 whole of the chambers are excluded, during the lifetime of the animal, from ex- 

 ternal influence, and are fiUed only by exhalations or secretions from the animal. 

 In the present instance, supposing the animal to have risen to the surface by 

 creating a partial vacuimi in the chambers, the presence of water therein is ac- 

 counted for by the fracture of the posterior part of the chamber of occupation 



* It would be advisable, in the event of another fortunate capture of the Nautilus, to lay open the 

 chambers under water, when the presence of gas in any of them would be ascertained, and it might 

 be received : the contents also of the central tube, if gaseous, would at once be detected. 



