726 



ZOOLOGY 



SECT. 



so close a character that successive turns are in immediate contact 

 and their walls fused together. But in Nautilus all the turns of 

 the spiral are in the same plane ; the spiral, in other words, is a 

 flat one, as has already been found to be the case in certain of the 

 Gastropoda (p. 686), whereas in Triton the spiral is an elongated 

 helix : in other words, the spiral of Nautilus is that of a watch- 

 spring, that of Triton that of a corkscrew. The possession by 

 Nautilus of the series of septa marking the position which the 

 animal has occupied at successive stages in its growth is another 

 striking difference. Moreover the relations of the soft parts of 

 the shell are radically different in the two cases. In Triton the 



s.n 



FIG. (537. Section of the shell of Nautilus pompilius, showing the septa (.?, s), the septal 

 necks (s. n., s. H.), the siphuncle, si. (represented by dotted lines), and the large .body-chamber 

 (c/i.). (From the Cambridge Natural History.) 



body is attached to the shell by the columellar muscle; in 

 Nautilus the main organic connection is by means of the siphuncle, 

 for, though it is chiefly through the pressure exerted by two great 

 lateral masses of muscle that the Nautilus retains its hold of the 

 shell, the muscular fibres are not attached to the latter in the 

 same intimate way as those of the columellar muscle of Triton. 

 Again, while the curvature of the body of Triton with the en- 

 closing shell is towards the ventral side, in Nautilus ic is towards 

 the dorsal. 



When the animal is removed from the shell it is found to 

 possess two regions, a distinct and relatively large, obtusely 





