EXTERNAL SHELL. 21 



There are no other organs of the body which are liable to periodic 

 changes in size ; and therefore, as the position and progressive 

 enlargement with age of the generative apparatus necessitates 

 results like those seen in the chambers and septa. I regard one 

 as the cause of the other." 



Prof. Keferstein decidedly agrees with Woodward and others, 

 that the adductor muscles of Nautilus continuously grow for- 

 wards, and are re-absorbed behind, rather than suddenly moved 

 as supposed by d'Orbigny. Then if these attachments added to 

 that of the continuous horny 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. In this case, 

 Mr. Seely's theory falls, and the animal simply builds the septa 

 as do the Cones, certain Bulimi, etc. 



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

 brane, 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 certainly 

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

 and probably prefers the sea-bed.* Still, the specific gravitj' 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 w r ith fluid from it. 



Woodward says (Man. Moll., p. 184): The use of the air- 

 chambers is to render the whole animal (and shell) of nearly the 

 same specific gravity with the water. Thus a Nautilus Pom- 



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

 that it has been commonly believed that the Nautilus occasionally rises 

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

 reported by Ilnmphins, but, after making continued and careful inquiries, 

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

 ever rises from the bed of the sea. Proc. Bost. 8oc.. N.IL, xii, 157, 1868. 



