14 EOCENE MOLLUSCA. 
required in the zonplicated shell, for the preservation of an animal, whose habitation, for 
the most part, is at a considerable depth, where the pressure of water is much increased, 
than in the plicated species, the peculiar construction of which would afford sufficient 
resistance, without that additional support which the smoother species may receive 
from this singular structure of the mantle. If, however, a necessity exist for the 
preservation of the shell in ordinary cases, how much more essential would it be that 
some compensating power should be possessed by an animal whose existence, in all 
probability, is dependent upon the buoyant principle of its partitioned shell; and how 
probable does it appear that this, an ordinary provision, should be employed for its 
protection. 
The tubular character of the siphuncle suggests an hydraulic action. To explain 
this, it is necessary to invest the animal with the power of emptying and filling the tube 
at discretion ; and this power it may be presumed to derive from the pressure upon the 
pericardial cavity, caused by the folding and contracting within the shell of the large 
cephalic mass. Under this pressure the fluid would be injected the whole length of 
the siphuncle, and, on the removal of the pressure, would return into the pericardium, 
to be there renovated and vivified with the other fluids, to be again injected when the 
animal returns within the shell. If the siphuncle had been a solid body, or composed 
of muscles, fibres, &c., it would have required to be permeated with arteries, blood- 
vessels, &c., for its sustenance; but by the simple process of the fluid returning into 
the body of the animal, all the complicated apparatus necessary to sustain a fleshy 
body is superseded ; circulation and renovation are accomplished, and the fluid is thus 
maintained in a condition capable of affording the nourishment to the shell which the 
present hypothesis requires. 
The theories here suggested are, as all other theories on the same subject must 
for the present be, merely speculative; for, to quote the observation of Professor 
Owen,” “much remains to be done before the theory of the chambers and siphuncle 
can rest on the sound basis of experiment and observation.” These alone will 
satisfactorily determine the real purposes of the membranous siphuncle; but, for my 
own part, I believe that the primary, and probably the only, function of that organ is to 
maintain the vitality of the shell, and that it may be looked upon as an elongated 
ceecum ; and that it is not, under any circumstances, used by the animal as a hydro- 
static balance. 
It is unnecessary here to particularise the various forms of external shells presented 
by the extinct tetrabranchiate Cephalopods, inasmuch as, of the numerous genera 
which swarmed in the ancient seas, only the Nautilus survived the secondary period. 
The dibranchiate Cephalopods, with the exception of the genus Aryonauta (which, 
with Bellerophon, constitutes Professor Owen’s family of testaceous Octopods), are without 
* Memoir on the Nautilus Pompilius, p. 47. 
