CEPHALOPODS. 



339 



SECTION OP SHELL OF TEARLT NAUTILUS 



(much reduced). 



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. macromphalus 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 crayfish. 



The genus Nautilus is of great antiquity, dating from an early 

 epoch in the Palaeozoic period, and forms the type of the family 

 Nautilidw, 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 Orthoceratidw, 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. 



Allied Families. 



