CEPHALOPODA. 11 



distance ; but the rupture is, in fact, required by the hypothesis, and the animal 

 would thus be deprived at once of all its means of attachment to the shell. Neither 

 by this periodic advance would the equilibrium of the specific gravity be maintained. 

 We are warranted, I think, in assuming that the specific gravity of the animal and its 

 shell, without the siphuncular aid, would be most nearly in cquilibrio with that of 

 the surrounding sea immediately after the formation of a new septum. Now the 

 growth of the animal would constantly tend to derange this equilibrium, until the 

 period should arrive for the formation of the new septum. The capability of the 

 animal, therefore, to rise and sink would be as constantly fluctuating, unless there 

 existed some mode of compensating for the increasing bulk of the body during the 

 interval between the formation of the penultimate septum and that of the last. This 

 compensation, however, would be provided in the case of a gradual advance of the 

 line of attachment ; for the vacated part of the dwelling-chamber, filled with exhala- 

 tions from the animal, and increasing in size as the body is advanced, would become 

 an air-chamber as effective as if it were inclosed by a new septum ; while, on the other 

 hand, a periodic advance of the muscles and cincture would deprive the animal of this 

 mode of maintaining the equilibrium. 



I have mentioned the rupture of the membranous tube, which would be the 

 consequence of the sudden advance of the body ; in fact, the hypothesis which attributes 

 to this tube the function of carrying off the water admitted into the vacated part of 

 the shell by the detachment of the cincture, requires, ex necessitate, that the tube 

 should be ruptured in order that the water should enter it ; and in that case the 

 membranous siphon in the deserted chambers would consist of detached fragments 

 extending from septum to septum, and which, having fulfilled their object and become 

 severed from the animal, would no longer retain vitality. This, however, is not the 

 fact. The membranous tube is continued entire through all the septa to the extreme 

 air-chamber,* and is a vascular organized substance, provided with an artery and a 

 vein for its nutrition ; and it maintains its vitality during the life of the animal. We 

 are compelled, therefore, to think that the function of the siphuncle must be coextensive 

 with the animal's existence. On these grounds, the theory suggested by M. d'Orbigny 

 is not more satisfactory than the hydrostatic theory which he rejects. Whatever the 

 function may be, it is evident that the air-chambers themselves would be as efficient a 

 float without the siphon as with it ; and the alteration of the specific gravity, as has 

 been stated, may and in all probability is, effected simply by the animal protruding or 

 withdrawing the cephalic mass from or into the dwelling-chamber of the shell, or, as 



* Professor Owen, speaking of the specimen of Spirula Peronii (fragilis), brought home by Sir Edward 

 Belcher, says: "On gently raising the exposed portion of the siphon with a needle, the soft siphon was 

 withdrawn, without sensible resistance, from the tube of the hard siphon ; the siphon so withdrawn must 

 have reached nearly to the innermost whorl. It exhibited a slight segmentation, answering to the suc- 

 cessively sheathed parts of the calcareous siphon." 



