THE PEARLY NAUTILUS. 185 



in comparatively shallow water, and the animals were so taken in the time of Rumphius, at the 

 end of the seventeenth century. Traps seem never to have been tried for them in deep water. 

 The fact that the living Nautilus was obtained from 320 fathoms shows that it occurs at great 

 depths. It is probably a mistake to suppose that it ever comes to the surface voluntarily to swim 

 about. It is probably only washed up by storms, when injured perhaps by the waves. The living 

 specimen obtained by us seemed crippled, and unable to dive, no doubt because it had been brought 

 up so suddenly from the depths of the sea." 



The shell of the Nautilus is involute or discoidal, and has but few whorls. The siphuncle, 

 instead of being placed near the inner margin of the convolutions, is nearly central. In the recent 

 Nautili the shell is smooth, but in many fossil species it is corrugated, like the patent iron-rooting 

 so remarkable for its strength and lightness. The umbilicus is small or obsolete in the typical Nautili, 

 and the whorls enlarge rapidly. In the Palaeozoic species the whorls increase slowly, and are sometimes 

 scarcely in contact. The last closed chamber is frequently shallower in proportion than the rest. 



In the recent Nautilus the mandibles are horny, but calcified to a considerable extent ; they are 

 surrounded by a circular fleshy lip, external to which are four groups of labial or lip tentacles, twelve 

 or thirteen in each group. They appear to answer to the 

 buccal membrane of the Calamary. Beyond these, on each 

 side of the head, is a double series of arms, or branchial 

 tentacles, thirty -six in number. The dorsal pair are ex- 

 panded, and unite to form the hood, which closes the 

 aperture of the shell, except for a small space on each side, 

 which is tilled by the second pair of arms. The tentacles 

 are lamellated on their inner sm-face, and are retractile 

 within sheaths, or " digitations," which correspond to the 

 eight ordinary arms of the Cuttle-fishes, their increased 

 numbers being indicative of a lower grade of organisation. 

 Besides these, there are four ocular tentacles, one behind INTERIOR OF THE SHELL OF PEARLY NAUTILUS, 

 and one in front of each eye. They seem to be instruments SHOWING THE LAST CHAMBER TO WHICH 



f ,. j> -r\ THE ANIMAL IS FIXED. 



of sensation, and resemble the tentacles of Doris and a>Mantle . 6)D( , rsaIFoId: ,, Sbell . muBCle . >siphuncle: ^ 

 Aplysia. On the side of each eye is a hollow plicated cCmk"' Hood:p>Te 

 process. This process bears the external ears. The 



cavity leads to the auditory capsule, along a passage lined with a glandular membrane. The 

 respiratory funnel is formed by the folding of a very thick muscular lobe, which is prolonged 

 laterally on each side of the head, with its free edge directed backwards into the branchial 

 cavity ; behind the hood it is directed forwards, forming a lobe, which lies against the black-stained 

 spire of the shell. Inside the funnel is a valve-like fold. The margin of the mantle is entire, and 

 extends as far as the edge of the shell ; its substance is firm and muscular as far back as the line of 

 shell-muscles and horny girdle, beyond which it is thin and transparent. The shell-muscles are united 

 by a narrow tract across the hollow occupied by the involute spire of the shell, and are thus rendered 

 horse-shoe shaped. The siphuncle, according to Owen, is vascular. It opens into a cavity 

 containing the heart (pericardium), and is most probably filled with fluid from that cavity. 



A\ ith respect to the purpose of the air-chambers, much ingenuity has been exercised in devising 

 an explanation of their assumed hydrostatic function, whereby the Nautilus can rise at will to the 

 surface, or sink on the approach of storms to the quiet recesses of the deep. Unfortunately for such 

 poetical speculations, the Nautilus appears on the surface only when driven up by storms, and its 

 sphere of action seems to be undoubtedly 011 the bed of the sea, where it creeps like a Snail, or perhaps 

 lies in wait for unwary crabs and shell-fish, like some gigantic Sea Anemone, with outspread tentacles. 

 The specimen dissected by Professor Owen had its crop filled with fragments of a small crab, and its 

 mandibles seem well adapted for breaking such shells as those of Crustacea, Echini, and Mollusca. 



^\Ir. Frederick Edwards* says : " It is obvious, therefore, that the hydrostatic balance would be 

 destroyed if any one of the deserted chambers were so injured as no longer to act as a float." 



In Woodward's " Manual of the Mollusca " we find it also stated that " the use of the air- 



* In his "Monograph of the Eocene Cephalopodous Mollusca" (Palseontographical Society, 1849, p. 12). 

 214 



