CEPHALOPODA. ‘13 
their terminal or, as in Spirula, their exposed position, are particularly lable to injury 
from the shocks caused by the retrogressive movements of the animals, we shall find 
that the hypothesis will be as applicable to them as to the external shells. In the 
extended series of observations made by Dr. Carpenter upon the microscopic structure 
of shells, it is shown by that gentleman that the outer covering or shelly mass of 
molluscous animals is invariably permeated by an organized membrane, and he says 
(Report Brit. Assoc., 1844, page 9): “I am much disposed to believe that in every 
distinct formation of shell substance there is a single layer of membrane, and I am 
further of opinion that this membrane was at one time a constituent part of the mantle 
of the mollusc.” He further represents this membrane to have, more or less, a cellular 
arrangement, the interstices of which are filled with carbonate of lime or inorganic 
matter; and, at page 10, he says: ‘‘ Coupling the appearances which I| have myself 
observed, with the observations of Mr. Bowerbank, on the formation of shell, and 
keeping in view the general doctrines of cell action which I have elsewhere endeavoured 
to develope, I am inclined to believe that these cells are the real agents in the pro- 
duction of the shell, it being their office to secrete into their own cavities the carbonate 
of lime supplied by the fluids of the animal.” He does not appear to have extended 
his researches so far as to determine whether any or what amount of vitality is 
possessed by these membranes; but from the continuity and intimate connexion of * 
this beautiful network, permeating the entire substance of the shell, we may imagine 
that some slight degree of vital existence pervades the whole membrane, by which it 
is possible that the inorganic material is preserved from disintegration. In those 
shells which appear to have been subject to the erosive action of acidulated waters, 
or other external agency, such as the apices of Cerithia and the umbones of Cyrene, 
the part most affected is that which is furthest removed from the main body of the 
animal ; while that part of the shell which is in close proximity with the mantle is 
not, or at least but little, altered; probably owing to its greater vitality, and to its 
being the part most essential for the protection of the animal. Not only is the entire 
formation of shelly matter permeated by an organic membrane, but in some of the 
shelly coverings of molluscous animals in the order Brachiopoda, there is a very 
peculiar structure, somewhat analogous with what has been before suggested as the 
especial use of the siphuncular tube in the Cephalopoda. Dr. Carpenter has pointed 
out that in most, or perhaps in all, of the nonplicated species of Terebratula, the whole 
surface of the shell is perforated with innumerable pores, into which are inserted 
vascular portions of the mantle of the animal, of a tubular form, and filled with fluid, 
which have no communication with the exterior, but are closed at-the outer surface of 
the shell, and occupy the entire space of the pores. These cecal appendages may be 
for the purpose of distributing a greater degree of vitality through the body of the 
shell; though for what especial purpose this provision is required in one group more 
than another, it is not easy to explain; perhaps a greater degree of strength is 
