THE SHELL. 35 



introducing Avater into it ; but the narrow calcareous covering 

 which partial!}^ confines this tube, preventing dilatation, militates 

 strong!}^ against this hypothesis. D'Orbigny's guess seems more 

 reasonable, that this tube may not only serve as an attachment, 

 but that it may also assist in the formation of a new septum, by 

 keeping filled with compressed air the vacant space, in the rear 

 of the animal, which is to be divided oif. Prof. Keferstein, of 

 Gottingen, supposes, also, that the Nautilus Pompilius, in order 

 to raise itself in its shell to the place where it designs constructing 

 a new partition, employs the tension of an aeriform fluid, which 

 it produces from the bottom of its sack, and which presses its 

 body upward. The air disengaged by the Nautilus develops a 

 considerable force, because it conquers not only the resistance 

 of weight of the animal itself, but also that of the weight of 

 about six atmospheres, which presses upon it in its habitual 

 station at the bottom of the sea. Prof. Keferstein decidedly 

 agrees with Woodward and others, that the adductor muscle of 

 Nautilus continuously grows forward, and is reabsorbed behind, 

 rather than suddenly moved as supposed by d'Orbigny. Then 

 if these attachments added to that of the continuous hornj'' girdle 

 suffice to hold the animal to its position, the septum, as Woodward 

 shows, would result from a rest-period, and should at least be 

 externally distinguishable among the growth-lines of the exterior 

 surface. 



It is only the backs of the septa which are covered with 

 membrane, and this membrane may be required to maintain the 

 vitality of the shell in the deserted chambers. Keferstein also 

 considers it most likely that the siphon serves to maintain the 

 purity of the air in the chambers. The Nautilus may swim, 

 being provided with a locomotive tube or siphon, but it certainlj^ 

 is not well adapted by its structure for that mode of progression, 

 and probably prefers the sea-bed.* Still, the specific gravity of 

 shell and animal so nearly equals that of the sea-water that 

 probably the additional displacement caused by spreading out to 

 the utmost its tentacles would cause it to rise in the water. It 

 is just possible that when the Nautilus issues from its shell the 

 gas contained in the last chamber in the rear of the animal may 

 expand, and so reduce the specific gravity. 



The siphuncle is vascular, it opens into the heart-cavity 

 (pericardium), and is most probably filled with fluid from it. 



* Mr. A. S. Bickmore, who collected Nautili at Amboina, observes that 

 it has been commonly believed that the Nautilus occasionally rises to the 

 surfiice, and "setting its sails, floats over the sea." This was first 

 reported by Rumjihius, but, after making continued and cai-eful inquiries, 

 he had satisfied himself that there is no reason to suppose that the animal 

 ever rises from the bed of the sea. — Proc. Bo^. Soc. A^. H., xii, 157, 1868. 



