MOLLUSC A. 



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 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 com- 

 pare the Nautilus shell with 

 that of some Gasteropods, for 

 there also chambers may be 

 formed. But these arise from 

 secondary alterations of an ori- 

 ginally continuous spiral, and 

 the resemblance is never very 

 striking. The fresh water snail 

 Planorbis has an unchambered 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 allied genera. 

 This list is usually swelled by the addition of the extinct 

 Ammonites, but there are some reasons for believing that 

 these belong to the cuttlefish section of Cephalopods. 



The following table states the chief points of distinction 

 between Nautilus and the other series of Cephalopods. 



FIG. 123. Section of shell 

 of Nautilus. (After LENDEN- 



FELD.) 



