20 EXTERNAL SHELL. 



introducing water into it ; but the narrow calcareous covering 

 which partially 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 off. Prof. Keferstcin, of Got- 

 tingen, supposes, also, that the JVflw//7f/x J'ftin/tili.H*. 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. 



In the Report of the Brit. Assoc. for 1804, Harry Seel} r says: 

 " On examining a Nautilus-shell, two large muscles are seen to 

 have been placed in the lower part of the body-chamber, and 

 connected round the involute spire by a narrow muscle an 

 arrangement to which the shell may owe its involute form. Be- 

 neath the muscles are the liver, which overlaps the spire, the 

 ovaries, which abut on a large part of the septum, and certain 

 digestive organs above thc.se. Before any new chamber can be 

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

 any increase in the ovaries eau take place, a place must he formed 

 behind. 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 the air-chambers, and. since the development of ova 

 would necessitate a forward growth of the mollusk, the discharge 

 of the ovaries would leave an empty space behind, into which the 

 animal could not retire, which would then be shut oil' b\ a sep- 

 tum moulded on the animal's body. In the male AV////-////S, the 

 tesles are placed in exact, ly the same position as the ovaries of 

 the female, and. excepting the liver, form the. largest organ in the 

 body, ll may therefore be concluded, that the development of 

 the male organs would produce results similar to those in the 

 other sex; and likewise end in the formiition of chambers. 



