CEPHALOPODA. 



429 



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 

 siph uncle which is in part cal- 

 careous. 



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 (Argonauta argo) 

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

 which bears a shell ; it is not chambered, and is a shelter 

 for the eggs a cradle, not 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 

 an originally continuous spiral. The Gasteropod shell is 



FIG. 232. Section of shell of 

 Nautilus. After Lendenfeld. 



