186 



NATURAL HISTORY. 



EXTEKIOR OF THE SHELL OF THE 

 PEARLY NAUTILUS. 



chambers is to render the whole animal and shell of nearly the same specific gravity with the water 

 in which it lives." But no such buoy would be required for a bottom -feeder ; indeed, it would prevent it 

 from remaining below. On the contrary, the facts of the case tend to show that, like the " water- 

 Spondylus," the chambers were filled, or partially filled, with sea-water, which must find its way into 

 the chambered portion of the shells, by endosmose, through its pores by the great pressure existing 

 at the depth at which the Nautilus is found (200 to 300 fathoms), thus displacing the air in spite of the 

 animal. Even at a depth of from twenty to thirty fathoms the pressure of such a column of water 

 would equal more than six atmospheres how much more, then, at three hundred fathoms 1 



Mr. George Bennett, M.R.C.S., F.L.S., through whom Professor Owen obtained the first specimen 



of the animal of the Pearly Nautilus described by him in 1832, 

 states : " On laying carefully open that portion of the shell which 

 contains the chambers, it was found to contain water, which of 

 course immediately escaped." 



The writer, in 1870, had an opportunity of opening the 

 chambered portion of the shell of a Nautilus umbilicatus, which 

 had been preserved, with the animal, in spirits of wine. The last 

 three chambers preceding that occupied by the animal were laid 

 bare for a distance equal to half the circuit of the shell-whorl. 



The siphuncle (when the chambers were laid open) was quite 

 entire, and sheathed in a thin nacreous investment, which, how- 

 ever, attains considerable thickness near to each septum. 



The chambers contained a large quantity of fluid, of which I 

 did not specially take note at the time; but on reading Professor 



Owen's memoir, I have no doubt that its presence in this, and also in Mr. Bennett's specimen, was 

 not abnormal (as I had supposed), but in accordance with the natural state of all camerated shells, 

 and that it is a misnomer any longer to call them " air-chambers." 



Professor Owen writes as follows : " From the extremity of the sac is continued a small tubular 

 membranous process, which passes through the siphonic apertures in the septa of the shell, and is con- 

 tinued, there is reason to believe, to the innermost chamber. 

 This tube has been surmised to be tendinous or muscular ; 

 but the attachment of the shell to the soft parts proves to be 

 effected by much more adequate means. Rumphius appears 

 to have been acquainted with its true structure, for he calls 

 it an artery (een langen ader), and, in fact, within the external 

 thin membrane is included a small artery or vein. How far 

 these vessels are continued within the chambered portion of 

 the shell, or in what manner they are distributed, remains 

 for some future investigation ; for in the present instance the 

 only part of the shell that was preserved was the small portion SECTION 

 adhering to one of the horny tendons, and the membranous 

 tube had been ruptured in removing the animal at a few 

 lines' distance from its origin at the mantle. This tube appears 



to be contracted at its origin, and its diameter at the wider part is one line and a half." 

 Even admitting that the purpose of the siphuncle is to maintain the vitality of the shell 

 during the long life of the animal, it seems difficult to imagine how this vitality can be maintained 

 in a non-vascular body. If the siphuncle be a means for repairing the shell, we ought to find some 

 connection between it and the shell, but such does not exist ; indeed, the fossil species have in 

 many instances enormously thick pearly or shelly siphuncles. 



In fact, when the shell of Nautilus, or of any other Mollusc, is once formed, it is extra-vascular, 

 or dead matter, in the same sense that nails and hoofs and hair of higher animals are so, being in- 

 capable of repair, save at the growing end, or where in contact with the shell-secreting mantle. 



In the specimen of Nautilus umbilicatus already referred to, which I had the good fortune to 

 examine, I observed the thin pellicle of membrane, described by Professor Owen, lining the chambers : 



OF THE SHELL OF THE PEARLV 

 NAUTILUS, SHOWING THE BODY-CHAMHER 

 AND THE SEPTA GIVING PASSAGE TO THE 

 SIPHUNCLE. 



