10 EOCENE MOLLUSCA. 



septum (en dessus de la derniere cloison) by two powerful muscles ; and the animal, 

 always increasing in bulk, must detach its body, and remove and place itself at a 

 determinate distance whenever it wishes to form a new partition. There must also 

 be left, between the penultimate partition and that which the animal is about 

 to construct, a space to be filled with air, while the animal is always under water." 

 And M. d'Orbigny therefore suggests " that the membranous tube and pericardial 

 cavity are required, when the new chamber is constructed, to empty the water 

 contained in it, and to fill it with air before the siphon entirely closes its wall in the 

 interior of the new air-chamber." This hypothesis does not appear to be more 

 satisfactory than the one involving a hydrostatic function. No allusion is made 

 in any way to the attachment of the animal to the shell by means of the horny 

 or epithelial cincture which, as we have seen, encircles the lower part of the body. 

 This cincture, in fact, hermetically closes the space between it and the last septum ; 

 and, unless it is detached, there would not be any external entrance through which 

 the water could penetrate, and the function, the object of the hypothesis, becomes 

 unnecessary. I cannot think therefore that this important attachment was overlooked ; 

 and I assume that M. d'Orbigny, when he says that the animal " must detach its 

 body," means that it must detach not only the adherent muscles which he mentions, 

 but also the horny cincture, to which he does not allude. Conceding this to be the 

 case, then, the hypothesis in question assumes that the advance of the body, pre- 

 paratory to the formation of the new partition, is not gradual ; but that the animal, by 

 sudden and nearly simultaneous efforts, detaches the adherent muscles and the 

 cincture, and removes its body to the necessary distance. In all other testaceous 

 molluscs, the advance of the adductor and adherent muscles is caused by the depo- 

 sition of new matter, by means of a thin membrane, part of the pallial membrane, 

 interposed between the extremity of the muscle and the inner surface of the shell. 

 The deposition is made, particle by particle, on the anterior part of the muscle, 

 portions of the posterior part probably becoming detached and absorbed ; but this 

 process is so gradual, that the attachment of the animal to the shell — an attachment, in 

 fact, necessary to its existence — is not affected ; and thus the muscle advances slowly 

 and imperceptibly. There does not appear to be any reason for supposing that a law 

 prevails among the cephalopodous molluscs different from that which regulates the 

 advance of the adherent muscle in the testaceous gasteropod. We may readily 

 conceive, on the contrary, that the entire detachment of the muscles and of the 

 cincture would be attended with considerable inconvenience to the animal ; for, in 

 that condition, the fulcrum or resisting power by which the animal is enabled to use 

 its tentacles and other organs efficiently, and which is essential to its existence, would 

 be temporarily lost. The sudden removal of the body forward would probably, 

 although it cannot be assumed that it would necessarily, cause the rupture of the 

 membranous siphon, for that organ may be sufficiently clastic to stretch to the required 



