THE PEARLY NAUTILUS. 183 



the laminated rudiment of a chambered shell in the Cuttle-fish; and the fully developed chambered 

 and siphunculated shell of the Belemnites and Spirula. 



ORDER II.- TETRABRANCHIATA. 



FAMILY VII. NAUTILIADJE. 



The external-shelled Cephalopods (represented at the present day by the " Pearly Nautilus" alone, 

 but in the past by the Nautilus, Ammonite, Goniatite, Orthoceratite, and a host of other forms) belong 

 to the Tetrabranchiata, or four-gilled division, and were once as extensively represented in the ancient 

 sca^ of our globe as the naked or internal-shelled Dibranchiata (two-gilled) division are in the seas of 

 to-day. 



With the exception of Spirula, already described (whose pearly shell is internal), Nautilus is the 

 only other siphonated shell now known amongst living Mollusca; the chambered character of the shell, 

 with its siphuncle, appears, therefore, to be a unique molluscan structure, entirely confined to the 

 Cephalopoda. 



In Nautilus the inner shell-layer and septa are nacreous (composed of mother-of-pearl), the outer 

 layer porcellanous (like porcelain). So far, then, as the composition of the shell goes, that of Nautilus 

 is the same as in many other Mollusca. Its coloration, which, when seen floating on the surface of the 

 water, "resembled a tortoiseshell cat" (as the sailors remarked), is good evidence of its being an external 

 shell; as is still more the fact that it is, when living, coated with a thin layer of epidermis, or 

 periostraciuii* which is not a living membrane, and can only be reproduced around the mouth of 

 the shell, or where it is within reach of the margin of the mantle, which is, in most Mollusca, the true 

 .shell-secreting organ. 



It is the umbonal t portion of bivalve shells and the spires of univalves which first become 

 eroded and injured; and one object, no doubt, in the formation of septa, or partitions, in all shells is 

 to shut off the damaged and untenantable part of their abodes. 



It is almost certain that the true " Pearly Nautilus " (Nautilus pompilius), as well as the "Paper 

 Nautilus " (Argonauta aryo), was known to the father of natural history, Aristotle (B.C. 350), for 

 ^ifter describing the Argonaut, he says : " But the other genus is in a shell, like a Snail ; it never 

 .quits its shell, but exists after the manner of a Snail, and sometimes outwardly extends its arms." 



No other notice of the Nautilus worthy of record occurs until the time of the old Dutch 

 naturalist, Rumphius (1705), who, during his long residence at Amboyna, was enabled to procure 

 ^specimens and make excellent observations thereon. 



The following is a translation} of the account given by this early observer, whose figure of the 

 : animul, as seen when taken out of the shell, is probably still (says Mr. Moseley) the best extant : 



" When the living Nautilus floats at the surface of the water it protrudes its head with all the 

 tentacles out, and spreads those out in the water, keeping the hinder part of the curl of the shell all the 

 while aboye water. On the bottom, however, the animal creeps with the other side uppermost, with 

 the head and tentacles on the bottom, and makes tolerably fast progress. The animal remains 

 mostly at the bottom, creeping sometimes into hoop-nets set for fish, and lobster-pots ; but after a 

 istorm, when the weather becomes calm, they are to be seen floating in troops on the surface of 

 the water. They are doubtless raised up by the waves caused by the storms. It follows that they 

 keep themselves together in troops on the bottom also. The floating, however, does not last long, 

 for, drawing hi all their tentacles, the animals turn their boats over and go down again to the bottom. 

 On the other hand, the empty shells are frequently to be found floating or cast up on the shore, for 

 the defenceless animals, having no operculum, are a prey to Crabs, Sharks, and Crocodiles; and 

 therefore the shells are mostly found with the edges bitten off. Since the animal does not adhere fast 

 to its shell, its enemies can easily drag it out, leaving the empty shell to float, 



; ' The young of this Nautilus, not larger than a Dutch shilling, are of a clean mother-of-pearl 

 .colour within and without. The rough shell substance overgrows the mother-of-pearl only after a 

 time, and this overgrowth commences from the foremost part of the boat." 



FremjMri, Gr., .upon, and ostraron, Gr., a shell : the layer of animal substance, or cuticle (cutis, the skin), which 

 Covers the outer surface of shells. 



t Umlo, iJnis, 1^., ,tke bpgs of a shield. J " D'Amboinsche Jlarikit kaiwer door": G. E. Rumphius, Amsterdam (1705), 



