410 



/ ' ) 'HL UM MOL I. USC A. 



it: 



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. Thus the compartments are not succes- 

 sive 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 Ceph- 

 alopod which has a shell at all 

 like that of the Nautilus is 

 Spirilla. 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) 

 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 is usually stated to be 

 formed by two of the arms, but it seems doubtful whether it 

 is not in reality due to the activity of the mantle. 



It is instructive also to compare the Nautilus shell with 

 that of some Gasteropods, for there also chambers may be 



FIG. 203. Section of shell of 

 nautilus. - - After Lenden- 

 feld. 



