CEPHALOPODA. 7 



Nautilus the animal is attached to the shell by two large lateral muscles, called the 

 adherent muscles, and by a belt or cincture of horny matter, which completely encircles 

 the posterior part of the visceral sac, and expands at the sides into broad discs, which 

 serve as the medium of insertion of the adherent muscles ; and the prolonged posterior 

 extremity of the visceral sac, forming the membranous siphuncle, is a third mode of 

 attachment. As the animal increases in size, the adherent muscles and the cincture 

 gradually advance their line of attachment, and the membranous tube at the same time 

 lengthening in proportion, a cavity is thus formed between the septum and the lower 

 portion of the visceral sac. A deposition of calcareous matter by the surface of the 

 mantle then takes place, commencing at the sides of the shell, and proceeding towards 

 the membranous tube, round which it is continued backward, and forms the calcareous 

 or testaceous siphon. Thus, as the animal increases in bulk, the dwelling-chambers 

 are successively formed and converted into air-chambers, by means of which the specific 

 gravity of the shell and its contents is maintained nearly in equilibrio with that of the 

 surrounding water. During the growth of the animal the anterior portion of the 

 mantle secretes calcareous matter, which it deposits in successive layers on the margin 

 of the aperture ; and thus the enlargement of the outer wall of the shell is effected. 

 I must add, that the theory of the gradual advance of the adherent muscles and the 

 cincture during the growth of the animal is opposed to the opinion of M. d'Orbigny, 

 who, in his hypothesis as to the function of the siphuncle, noticed subsequently, 

 maintains in effect, that the advance of the muscles (and, I presume, of the cincture 

 also) is periodic. 



The Argonaut presents an extraordinary deviation from the general laws which 

 govern all other molluscous animals ; inasmuch as the animal, although perfectly free 

 and unattached to the shell it inhabits, is not now considered to be a mere parasite/ 



* It is foreign to the present purpose to enter into the question as to the parasitism of the Ocythoe ; 

 the experiments of Madame Jeannette Power, confirmed to a great extent by the observations of M. Sander 

 Rang and M. d'Orbigny, and more recently by those of Mr. Adams, daring the voyage of H.M.S. Samarang, 

 are generally considered as removing all doubt as to that animal being the fabricator of the shell in which 

 it is found ; and the theory of parasitism is now rejected by nearly all naturalists. A detailed account of 

 the facts ascertained and recorded by Madame Power and M. Rang will be found in M. Rang's Memoire, 

 published in Guerin's ' Magasin de Zoologie,' and in Madame Power's 'Observations on the Poulpe of the 

 Argonaut,' translations of which are published in the 'Mag. Nat. Hist.,' new series, vols, iii andiv. The 

 observations of Mr. Adams, published in the ' Zoology of the Voyage of the Samarang,' tend to prove that the 

 shell is constructed by the female Argonaut as a nest for receiving her eggs, and protecting them from injury, 

 resembling in some measure the rudimental capsules secreted by many marine Gastropods for the preser- 

 vation of the embryo. The animal firmly retains possession of this light calcareous shell-nest by means of 

 the broad expanded membranes of the posterior pair of tentacles ; but when disturbed or captured, she 

 loosens her hold, and leaving her cradle to its fate, swims about independent of her shell. Having once 

 deserted the nest, it appears that she has not the power, or more properly the sagacity, to re-enter it. 

 Numbers of male Argonauts were taken by Mr. Adams, but always without shells. There are, however, 

 in this theory, difficulties which probably future observation may explain. In the first place, the shells 



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