12 EOCENE MOLLUSCA. 
M. de Blainville has shown to be the case in Spirula, within a cavity formed by the 
anterior extremity of the mantle. We may reasonably infer, therefore, that to assist 
in varying the specific gravity is not the principal function of the siphuncle. But, in 
any view, the preservation of the deserted chambers, as air-chambers, is essential to 
the motive power of the animal; for it is only by their tendency to float when the 
cephalic mass is protruded, that the animal is enabled to rise; and this nicely- 
adjusted counterpoise is maintained, as we have seen, by the addition of new air- 
chambers, as the animal and the shell increase in size. It is obvious, therefore, 
that the hydrostatic balance would be destroyed if any one of the deserted chambers 
were so injured as no longer to act as a float. Now it is known that the shells of the 
testaceous molluscs are not wholly inorganic substances ; but that a vital communica- 
tion is maintained between them and the animals, and that where this communication 
ceases, the deserted whorls of the shell lose their vitality and become brittle ; the 
calcareous matter falls off in particles, and the shell is much more susceptible of 
injury. In Bulimus decollatus (Helix decollata, Linn.) and other similar shells, in which 
the earlier whorls are wholly deserted, the animal on withdrawing its body forms 
behind its extremity a concave septum. In these cases the apex of the shell, no longer 
necessary, is easily broken off; in which state the shell is said to be decollated.* In 
the siphoniferous shells, however, the preservation of the chambers, as air-chambers, 
is, as we have already seen, essential to the motive power of the animal. It is true 
that in the Nautilus, the mode of convolution, upon a vertical axis, is admirably 
adapted to strengthen and protect the first-formed volutions; but in shells not so 
constructed, and even in those possessing the nautiliform mode of convolution, it 
would appear to be essential that the vitality should not be lost. How, then, is the 
necessary communication between the animal and the air-chambers maintained, and 
the vitality of the deserted shell preserved? It has been shewn that the siphuncle 
traverses the chambers to the extreme nucleus of the shell, and that it is provided 
with a small artery and vein; and we also learn from Professor Owen’s Memoir, that 
in the Nautilus “a delicate pellicle, distinct from the tube, is continued over the 
outer part of the testaceous tube, and also over the whole inner surface of the chamber.” 
May we not then reasonably regard the siphuncle with its artery and vein, and the 
pellicle lining the air-chambers, as the organs destined to maintain the vitality of the 
shell, and feel ourselves justified in considering this office to be in fact the primary 
function of the siphuncle ? And when we bear in mind that the internal shells, from 
* Tam wholly indebted to my friend Mr. Searles Wood for the following theory as to the siphuncular 
function, and the main arguments in support of it. The well-known conchological attaimments of that 
gentleman exact respect for every opinion of his on subjects like the present; but independently of this, 
the theory itself seems to me to be far more probable than any hitherto advanced as to the office of the 
siphon ; and I therefore gladly avail myself of Mr. Wood’s permission to introduce his views of the subject 
into my text. 
