122 THE NAUTILUS. 



tation and contraction.* If we adopt this theory, we may 

 suppose that the contained air is a secretion of the artery 

 which is continued down the membranous tube, as proved 

 by Mr. Owen's dissections ; and it may afterwards escape 

 by a communication which exists between the tube and 

 branchial cavity through the medium of the pericardium. 

 " But," says Professor Owen, " 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 indis- 

 pensable ; 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."-}- These objections, fatal perhaps to Parkinson's 

 views, Dr. Buckland attempted to obviate by a modifica- 

 tion of Hooke's theory. Assuming that the chambers are 

 air-receptacles, the Doctor conjectured that when the ani- 

 mal is at the surface and wishes to sink, it forces into the 

 siphuncle a quantity of water previously contained in the 

 pericardium or bag enclosing the heart. The consequent 

 distention of the siphuncle compresses the air in the cham- 

 bers, and the bulk of the exterior of the body being thereby 

 diminished, its specific gravity is increased and it sinks. 

 When it wishes to rise it has only to withdraw the pres- 

 sure from the pericardium ; the elasticity of the air in the 

 chambers forces the water back from the siphuncle into the 

 external cavity, and thus by increasing its total bulk renders 

 the specific gravity again less than that of the water. 

 The main objection to this hypothesis is derived from the 

 structure of the syphonal tube, which seems to be little 

 capable of dilatation in any species of Nautilus, or many- 

 chambered molkisk ; and, in some species, is so coated 

 over with a calcareous incrustation as to render nuich 

 swelling of it altogether impossible. Perhaps, as Professor 

 Owen states, nothing more is necessary to enable the Nauti- 

 lus to rise than the full unfolding of its organs and their 

 protrusion from the shell. Buoyed by the expanded organs, 

 the animal has only to yield itself to the upward force de- 

 rived from the shell being specifically lighter than the medium 

 which surrounds it, for we assume that the chambers of the 

 shell are filled successively with a light gas in the course of 

 their formation. After his notice of the structure of the 



* Outlines of Oryctology, 167. t Owen's Memoir, 4f!, 47. 



