THE PEARL Y NA UTIL US. 341 



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 hitching itself 

 forward in 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. The compartments seen on a divided shell 

 are not exactly successive chambers, they are fractions of 

 successive chambers abandoned and partitioned off as more 

 space was gained in front. Moreover, all the compartments 

 are in communication by a median tube or 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 quies- 

 cence, 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." 



The only other living Cephalopod which has a shell 

 like that of the Nautilus is Spirula. In this form the shell is 

 again chambered and spirally coiled in one plane. But it is 

 without a siphuncle, and lies enveloped by folds of the mantle. 



There can be no confusion between the beautiful shell of 

 the cuttlefish called the paper Nautilus {Argonauta argo) 

 and that of our type. For it is only the female Argonaut 

 which bears a shell, it is made by two of the arms not by 

 the mantle, it is not chambered, it is a shelter for the eggs — 

 a cradle not a house. 



It is instructive also to compare the Nautilus shell with 

 that of some Gasteropods, for there also chambers may be 

 formed. But these arise from secondary alterations of an 

 originally continuous spiral, and the resemblance is never 

 very striking. The fresh-water snail Planorbis has an un- 

 chambered shell spirally coiled in one plane, but in this and 

 in similar Gasteropods, the foot is turned towards the internal 

 curve of the coil, while that of Nautilus is directed externally. 



There are only about half a dozen living species of 

 Nautilus, but there are many hundred fossils of this and 



