NAUTILUS. 



411 



formed. But these arise from secondary alterations of an 

 originally 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 



FIG. 204. TheJPearly Nautilus (Nautilus pompilius}. 

 After Owen. 



The shell is represented in section, but the animal is not dissected. 

 c. , Last or body-chamber, separated by a septum (se.) from the 

 compartment behind ; s., the siphuncle traversing all the 

 compartments ; in., the portion of the mantle which is reflected 

 over the shell ; /i., the hood ; e., the eye with its opening to 

 the exterior; /., the lobes which bear the sheathed tentacles 

 (*".) ; s'., the incomplete siphon ; mu, the shell muscle; ., the 

 position of the nidamental gland. 



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 



