484 



PHYLUM MOLLUSCA 



an originally continuous spiral. The Gasteropod shell is 

 usually unsymmetrical, and the foot {ventral) is turned 

 towards the internal curve of the coil, while in Nautilus 



Fig. 275. — The Pearly Nautilus {Nautilus pompilius). 



— After Owen. 



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

 Part of the mantle has been removed, c. Last or body 

 chamber, separated by a septum (se.) from the compartment 

 behind ; s., the siphuncle traversing all the compartments ; 

 m., the portion of the mantle which is reflected over the shell ; 

 h., the hood ; e., the eye with its opening to the exterior ; 

 /., the lobes which bear the sheathed tentacles {t.) ; si., the 

 incomplete siphon ; mu., the shell muscle ; n., the position 

 of the nidamental gland. 



the dorsal part of the animal is towards the internal surface 

 of the chamber. 



There are only some three or four 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 



