382 



MOLLUSCA. 



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 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 

 ;^ v • , off a part of the chamber in 

 1 1 Wp|K^ which it has been living. Thus 

 §§j||PI;Jg5^| the compartments are not suc- 

 cessive chambers, but fractions 

 of successive chambers, aban- 

 doned and partitioned off as 

 more space was gained in front. 

 All the compartments 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." 



The only other living Cephalopod which has a shell at all 

 like that of the Nautilus is Spirula. In it 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) 



Fig. 167. — Section of shell of 

 nautilus. — After Lenden- 

 feld. 



