NAimLus.- 



-MOLLUSCA.- 



-NAunLus. 



317 



Cultle-fishes. In addition to these there are four ocular 

 tentacles, which Professor Owen imagines to be instru- 

 ments of sensation, like the tentacles of the Dorides. 

 The dorsal pair of the brachial tentacles are expanded, 

 and unite to form a hood which closes the aperture of 

 the shell, corresponding to the shell-secreting saj7« of the 

 argonaut. This hood appears also to serve the animal 

 as an organ of locomotion or kind of foot, analogous 

 in action to the foot of the Gasteropods. The shell of 

 the Nautilus is involute or discoidal, and consists of 

 few whirls. Internally, it is divided into cells or 

 chambers, by a series of partitions or sejHa, and they 

 are connected together by a tube or siphuncle. This 

 siphuncle is membraneous, with a very thin nacreous 

 investment, and is usually central. The last chamber 

 is capacious, the cavity being twice as large as all the 

 others put together. It is occupied by the animal, but 

 the rest are empty, and are known as air chambers, and 

 are lined by a very thin membrane. The use of these 

 air chambers appears to be to render the shell, with its 

 inliabitant, of nearly the same specific giavity with the 

 water. " A Nautilus pompiUus (in the cabinet of Mr. 

 Morris)," says Mr. Woodward, " weighs one pound, and 

 when the siphuncle is secured, it floats with a half 

 pound weight in its aperture. The animal would have 

 displaced two pints (two and a half pounds) of water, and 

 therefore if it weighed three pounds, the specifio gravity 

 of the animal and shell would scarcely e.xceed that of 

 salt water." The object of the numerous partitions is 

 probably more for the purpose of guarding against the 

 collisions to which the shell is exposed, than for sus- 

 taining the pressure of the water as has been surmised 

 by some authors ; and the purpose of the siphuncle 

 lias been suggested by Mr. Searles Wood to be that of 

 mauitaining the vitality of the shell during the long 

 life these animals most likely enjoy. The sexes in the 

 animals of this order are no doubt distinct, but all the 

 specimens that have as yet been examined of the living 

 Nautilus have been females. We are left to conjecture, 

 therefore, how far the dilfcrences observable in the 

 shells, and which have been made by conchologists of 

 specific value, are dependent upon sex. Dr. Melville 

 has been led to the conclusion from a study of these 

 shells, that those individuals which are umbilicated are 

 the males, and thuse without the umbilicus are females. 



THE NAUTILUS POMPILIUS— fig. 211— is the only 

 species the living inhabitant of which has been seen, 

 and as these have all been females, and the shells are 

 all imperforate, it has been suggested that the umbili- 

 cated species, N. macromphalus, is the male. A 

 wider range of observation, however, than can be easily 

 obtained, would be requisite to enable us to pronounce 

 with certainty on the subject. In a case so out of the 

 reach of common observation, and in which so many 

 circumstances concur to limit or baffle inquiry, it 

 would be little in keeping with the spirit of science to 

 give other than a probable opinion. 



Our knowledge of the habits of the Nautilus is very 

 limited. Most probably, like the other cuttle-fishes, 

 it feeds upon Crustacea and shell-fish. As we have 

 mentioned before, the animals are not in the habit of 

 swimming ; they creep upon the ground at the bottom 

 of the sea. They are said, by means of their air 



chambers, to rise at will to the surface and sink again 

 on the approach of storms to the quiet recesses of the 

 deep. They are also said to reside habitually at great 

 depths in the ocean ; but as the partitions or Sf2ita 

 appear to be formed periodically, and as the Nautilus 

 must, consequently, have frequently an air cavity 

 between it and the shell, it is evident that these 

 chambered cephalopods could not exist in very deep 

 water. Empty bottles, we fiud, if securely corked, and 

 sunk with weights beyond one hundred fathoms, are 

 always crushed ; and therefore these shells with their 



Fig. 211. 



Nautilus Ponipiliiis. Shell with animal. 



air chambers, are probably limited to a depth of twenty 

 or thirty fathoms at the utmost. The specimens which 

 have been taken alive, have been captured at the 

 surface ; and though they can undoubtedly swim by 

 njcans of their respiratory jets, like other cuttle-fish, 

 yet the form of their shell must be ill calculated for 

 that method of progression. Their sphere of action is 

 on the bed of the sea, where they creep like a snail, 

 only mounting to the surface when driven up by storms. 

 Humphius was the first who described the animal of 

 the Nautilus. " When he floats on the water," he says, 

 " he puts out his head and all his barbs (tentacles), 

 and spreads them upon the water, with the poop (of 

 the shell) above water; but at the bottom he creeps 

 in the reverse position, with his boat above him, and 

 with his head and barbs upon the grotmd, making a 

 tolerably quick progress. He keeps himself chiefly 

 upon the ground, creeping sometimes also into the nets 

 of the fibhermen; but after a storm, as the weather 

 becomes calm, they are seen in troops floating on the 

 water, being driven up by the agitation of the waves, 

 whence one may infer, that they congregate in troops 

 at the bottom. This sailing, however, is not of long 

 continuance ; for having taken in all their tentacles 

 they upset their boat, and so return to the bottom." 

 By what mechanism this ascent and descent of the 

 Nautilus is efi'ected is still a matter of conjecture. 

 Professor Owen, whose " Memoir upon the Pearly 

 Nautilus," is described by Dr. Johnston "as one of 

 the best and most beautifid monographs in compara- 

 tive anatomy," thinks that nothhij; more is necessary 



