EXTERNAL SHELL. 21 



There arc no other organs of tlic bod^- which are lial)le to i)erio(lic 

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

 enlargement witli age of tlie generative apparatns necessitates 

 I'esults like those seen in tiie chanilicis and septa. I regard one 

 as the cause of the other." 



Prof. Keferstein decidedly agrees with Wooihvard and otliers, 

 that the adductor muscles of Nautilus continuously grow for- 

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

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

 that of the continuous horny girdle suflice 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 onl}' the backs of the septa which arc 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 probabl}^ 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 tiie 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 Avascular, it opens into the heart-cavitv 

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



Woodward says (Man. Moll., p. 1.S4): 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 Na/utilui^ Fom- 



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

 that it has been commonly believed tliat tlie Xaiitilus occasionally ri.ses 

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

 reported by Rumphius, l:)ut, after making continued and careful inquiries, 

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

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



