THE PEARLY NAUTILUS 



483 



It is called " pearly " on account of the appearance of the 

 innermost layer of the shell. This is exposed after the soft 

 organic stratum and the median porcellanous layer which 

 bears bands of colour have been worn away, or dissolved 

 in a dolphin's stomach, or artificially treated with acid. 



The beautiful shell is a spiral in one plane, divided into 

 a set of chambers, in the last of which the animal lives, 

 while the others contain gas. The young creature inhabits 

 a tiny shell curved like a horn ; it grows too big for this, 

 and proceeds to enlarge its dwelling, meanwhile drawing 

 itself forward from the older 

 part, and forming a door of 

 lime behind it. This process 

 is repeated again and again ; 

 as an addition is made in front, 

 the animal draws itself forward 

 a little, and shuts off a part of 

 the chamber in which it has 

 been living. All the compart- 

 ments are in communication 

 by a median tube of skin — the 

 siphuncle — which is in part 

 calcareous. 



It has been suggested that 

 " each septum shutting off an 

 air - containing chamber is 

 formed during a period of 

 quiescence, probably after the 

 reproductive act, when the visceral mass of the Nautilus 

 may be slightly shrunk, and gas is secreted from the dorsal 

 integument so as to fill up the space previously occupied by 

 the animal." 



There can be no confusion between the beautiful shell of 

 the cuttlefish called the Paper Nautilus {Argonaiita argo) 

 and that of our type. For only the female Argonaut bears 

 a shell ; it is not chambered, and is a shelter for the eggs 

 — a cradle, more than a house. It seems to be formed by 

 two of the arms. 



It is instructive to compare the Nautilus shell with that 

 of some Gasteropods, for there also chambers are occasion- 

 ally formed. But these arise from secondary alterations of 



Fig. 274. — Section of shell of 

 Naxitilus. — After T.endenfeld. 



