'20 EXTERNAL SHELL. 



introducing water into it ; but tiio narrow calcareous covering 

 which partially confines this tube, preventing dilatation, militates 

 strongly against this hypothesis. l)'Orl>igny'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 tlie vacant space, in the rear of 

 the animal, which is to be divided off. Prof. Keferstein, of Got- 

 tingen, supposes, also, that the N(iuf.ilii.'< Pompilius, in order to 

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

 a new partition, employs tlie tension of an aeriform fiuid, 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 Iiabitual station at 

 the bottom of tlie sea. 



In the Report of the Brit. Assoc, for 18(54, Harry Seely says: 

 '• On examining a Naufil h.^-hIigW, two large muscles are seen to 

 have been phiced in the lowcu' [Kirt of th(> bod^'-chamber, and 

 connected round the involute spire by a narrow muscle — an 

 arrangement to which the sliell may owe its involute lonn. Ue- 

 neath the muscles are the liver, which overlaps the s[)ire, tlie 

 ovaries, whicli abut on a large part of tlie septum, and certnin 

 digestive organs above these. Before any new chamber can be 

 made, the shell-muscles must have moved forward ; and before 

 any increase in the ovaries can take place, a place must be formed 

 licliind. As the animal steadily grows, all its organs would 

 enlarge ; and, with each successive brood, the distended ovaries 

 would require more space. There is a similar gradual increase 

 in the size of tlie air-chambers, and, since the development of ova 

 would necessitate a forward growth of the mollusk, the discharge 

 of the ovaries would leave an em})ty space behind, into which the 

 animal could not retire, which would then be shut olf by a sep- 

 tum moulded on the aniinaPs bod}'. In the male NaiUiliis^ the 

 testes are placed in exactly the same position as the ovaries of 

 tlie reiiiide, and, excepting the liver, form the largest organ in the 

 itodv. It may therefore be concluded, that tlu^ development of 

 the male organs would i)r()(ln(M' results similar to those; in the 

 other sex: and likewise end in tlie (brniiition of chambers. 



