308 CEPHALOPODA. 



Externally, this shell presents nothing remarkable 

 except the elegance of its shape ; but on making a 

 section of it, as represented in Fig. 243, its cavity is 

 found to be partitioned off, by numerous shelly plates, 

 into various chambers, in the last and largest of 

 which the body of the animal is lodged. A long 

 tube, or siiiiJiuncle, partly calcareous, partly mem- 

 branous, passes through all the compartments quite 

 to the end of the series. This membranous siphuncle 

 is continued into the animal, and terminates in a 

 cavity contained within its body, which is in free 

 communication with the exterior. 



Various conjectures have been indulged in relative 

 to the end answered by this chambered condition of 

 the shell. It has been suggested that the chambers 

 might be filled with air generated by the Nautilus, 

 and thus made so buoyant, that the specific gravity of 

 the animal should nearly correspond with that of the 

 surrounding medium, and that acting in the manner 

 of the swimming bladder of a fish, the creature would 

 float or sink, as the contained air was alternately 

 rarefied or compressed. Should this supposition be 

 true, it would ^em probable that the simple re- 

 traction of the muscular head into its shell would 

 cause the needful compression of the air in this 

 singular float, and allow the Nautilus to sink to the 

 bottom, while- the protrusion of its arms, by taking oif 

 the pressure, and thus allowing of the expansion of 

 the confined air, would give every needful degree of 

 buoyancy, even sufficient to permit the mollusk to 

 rise like a balloon to the top of the sea. 



The characteristic feature in the Nautilus is the 

 conversion of the sucker-bearing arms of other 

 Cephalopods into an apparatus of sensitive tentacula, 

 quite destitute of suckers. Its gills or branchiae are 

 four in number instead of two, the head is covered 

 with a strong leathery hood, which, when the animal 

 retires into its shell, closes the orifice like a door. 

 In place of the eight sucker-bearing arms of the 

 Poulpe, there are forty tentacular appendage^S, which 



