NAUTILUS. 



229 



all the Cephalopods is the Nautilus, the inhabitant of a chambered shell, 

 which is sufficiently common 



" A shell of ample range, and light 



As the pearly car of Amphitrite, 



"Which sportive dolphins drew." 



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 siphuncle, partly calcareous, partly membranous, 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. 



FIG. 243. PEARLY NAUTILUS (wi.h tJie sJiell in section}. 



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, 

 tli at 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 

 seem probable that the simple retraction 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 ot its arms, by talcing 

 off 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, 



