CEPHALOPODS. 
339 
The function of the siphuncle probably is to preserve the vitality of the first formed 
portion of the shell, which without some such means of preservation would be 
liable to decay. The animal is somewhat feebly attached to the shell by two large 
adductor muscles one on each side of the body, which are, as it were, connected by 
a muscular girdle of the mantle passing round the body from muscle to muscle. 
The chambered shell is beautifully pearly within, but has an external porcellaneous 
coating. A full-grown shell has about thirty-six septa, which are relatively 
equidistant, showing that the growth of the animal is regular and gradual 
throughout life. The septa give immense strength to the shell, sufficient to 
resist the pressure of the water at great 
depths upon the air-chambers between them. 
These air-chambers undoubtedly serve to 
buoy up the shell when the animal is swim¬ 
ming or desires to rise to the surface; but 
the old stories of its filling' the cells at 
pleasure with either air or water, and so 
rising to the surface or descending to the 
bottom, are mere fables, and comparable to 
the legends respecting the sailing of the 
argonaut. The shells of the male and 
female are said to present certain slight 
differences. Very little is known of the 
habits and economy of the pearly nautilus, 
but, as already remarked, it is most likely a deep-water animal, as a rule living 
at depths far beyond the action of storms. It would probably be obtained by 
dredging or by means of baited traps. A specimen dredged off the Fiji Islands, at 
a depth of about three hundred and twenty fathoms, was kept alive for some 
time in a tub of sea water. The mode of growth of the nautilus has been a 
subject of much discussion, and the way in which the successive air-chambers and 
septa are formed is not known with certainty. The living forms of Nautilus 
probably belong to three distinct species. N. pompilius has a wide distribution 
in eastern seas, specimens having been obtained in the Indian Ocean (Andaman 
Islands), at the Moluccas and Java, and in the Pacific at the New Hebrides and 
Fiji; N. umbilicatus is recorded from the Solomon Islands, and New Ireland; 
and N. macromplialus from New Caledonia and the Isle of Pines. The animal of 
Nautilus is used as an article of food among the natives of the New Hebrides, 
New Caledonia, and Fiji, it being captured by the Fijians in traps baited with 
boiled crajffish. 
The genus Nautilus is of great antiquity, dating from an early 
' epoch in the Palaeozoic period, and forms the type of the family 
Nautilidce, which includes several extinct genera. There are allied extinct 
families, collectively forming a group characterised by the simple structure of the 
septa of the shell, such septa having their concavities directed towards its aperture. 
Among these, the Orthoceratidce, as typified by the Palaeozoic genus Orthoceras, 
may be characterised as unrolled nautili, the shell—which sometimes reaches an 
immense length—forming a long cone. 
SECTION OF SHELL OF PEARLY NAUTILUS 
(much reduced). 
