CEPHALOPODA. 13 



their terminal or, as in Spirilla, their exposed position, are particularly liable 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 arc 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. Bowcrbank, 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 Cyrena, 

 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 caecal 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 



