THE "SAILING" OF THE NAUTILUS. 269 



feet, but, being highly flexible, are adapted for employment 

 also as prehensile arms, with which their owner captures 

 its prey, and they are rendered more perfect for this purpose 

 by being furnished with suckers which hold firmly to any 

 surface to which they are applied. The Cephalopods 

 which have the foot divided into ten of these segments or 

 arms are called the Decapoda y those which have only eight 

 of them are called the Octopoda. All of these have two 

 plume-like gills one on each side and so are called 

 Dibranchiata ; and in the eight-armed section of these is the 

 argonaut or Paper Nautilus. Of the Pearly Nautilus and 

 the four-gilled order to which it belongs I shall have more 

 to say by-and-by : at present we will follow the history of 

 the argonaut. 



Notwithstanding all that has 

 been written of it, it is only 

 within the last fifty years 

 that this has been correctly 

 understood. An eight-armed 

 cuttle was recognised and named 

 Ocytkoe, which, instead of hav- 



,., ,, FIG. 20. THE PAPER NAUTILUS 



ing, like the common octopus, (Arsomula argo) RETRAC . 

 all of its eight arms thong-like TED WITHIN ITS SHELL. 

 and tapering to a point, had 



the two dorsal limbs flattened into a broad thin mem- 

 brane. Although this animal was sometimes seen dead 

 without any covering, it was generally found contained in 

 a thin and slightly elastic univalve shell of graceful form 

 and bearing some resemblance to an elegantly shaped boat. 

 It did not penetrate to the bottom of this shell ; it was not 

 attached to it by any muscular ligament, nor was the shell 

 moulded on its body, nor apparently made to fit it. Hence 

 it was long regarded as doubtful, and even by naturalists so 



