CEPHALOPODA. 9 
maintains that a fluid is contained in the pericardium, the position of which is 
alternately changed from that cavity to the siphuncle ; and that in this shifting fluid 
the hydraulic balance consists, the chambers being filled with air alone, the elasticity 
of which would admit of the alternate expansion and contraction of the membranous 
siphuncle. Prof. Owen has pointed out objections to both these hypotheses. The only 
organ apparently by which the gaseous fluids of Mr. Parkinson’s theory can be 
secreted, is a small artery continued down the siphon, but which would not be adequate 
for the purpose ; and the form and size of the siphon would not allow of an escape of 
gas so free as to make the consequent sinking of the shell sufficiently rapid for 
defensive purposes. In some extinct species of Vawti/us the membranous siphuncle 
appears to have been capable of considerable dilatation, instances of which are 
mentioned by Dr. Buckland; but Professor Owen states that, in all the specimens he 
had examined, the membranous siphuncle, after the first chamber, presented an 
inextensible and almost friable texture, and was coated beyond the extremity of the 
testaceous siphon with a thin calcareous deposit; and that, in certain extinct species, 
the testaceous or calcareous siphon extended from septum to septum, rendering a 
dilatation of the membranous tube physically impossible. The calcareous siphon of 
the recent Spirula, as is well known, exhibits this form of structure. It is ascertained 
that, by the conversion of the dwelling-chambers of the animal into what may 
be termed air-chambers, the specific gravity of the Nautilus, and of its shell, 
may be maintained nearly in equilibrio with that of the sea. This equilibrium 
would be very sensibly affected by the position of the body of the animal with 
reference to the shell ; and Professor Owen therefore inclines to the opinion® that the 
variation of the specific gravity is caused chiefly by changes in the extent of the surface 
exposed to the water, according as the body may be expanded beyond the aperture of 
the shell, or more or less withdrawn within the dwelling-chamber. At the same time it 
is not improbable that the siphuncle, filled with the fluid propelled into it from the peri- 
cardium, in consequence of the pressure caused by the contraction of the animal within 
the shell, may assist in affecting the specific gravity; it certainly, however, does not 
appear to be capable of varying the specific gravity of the shell sufficiently for the 
wants of the animal, and that function, if attributable to it, must consequently be 
merely secondary. I am therefore inclined to agree with M. d’Orbigny, who rejects 
the supposition that the action of the siphuncle is hydrostatic. That naturalist assigns 
to the membranous tube which enters the calcareous siphon, and communicates with 
the pericardial cavity, a function widely different, and only to be called into action 
when the animal constructs a new air-chamber. ‘ On this occasion,” he says, ‘‘ many 
difficulties have to be overcome; the extremity of the body is attached above the last 
* In this opimion M. de Blainville concurs (Mémoire sur Animal de la Spirula, &c.), and he shows 
that a similar mode is used by the Spirula; the alteration of the specific gravity being effected by the with- 
drawal of the cephalic mass into a cavity formed by the upper portion of the body. 
0) 
