222 MOLLUSCA. 



animal through the medium in which it is to move." 

 By-and-by, as if tired with its exertions, it sunk ; 

 and after a while, fixing its suckers on the bottom, 

 it began to rear up its shell and crawl on its head,* 

 (somewhat in the manner of a snail,) the arms turned 

 upwards, and the sails spread over the shell. f 



Nautilus, J the Pearly Nautilus. 



Somewhat resembling the preceding in general 

 form, the genus before us inhabits a large and strong 

 shell, with a smooth rounded outline, somewhat flat- 

 tened at the sides, with a very wide mouth, and 

 with the back part rolled into a spiral within the 

 cavity. Its great peculiarity is, that it is divided 

 within into a great number of separate chambers 

 with shelly walls, in the outermost of which, of 

 course, the animal resides. A slender tube runs 

 through the centre of all the chambers, quite to 

 the end of the spire. The membranous end of this 

 tube is continued into the body of the living animal, 

 which is supposed to have the power of admitting 

 water into it or ejecting it, and thus making itself 

 either heavier or lighter than the surrounding me- 

 dium. The inhabitant of this remarkable shell dif- 

 fers in many important points from other Cepha- 

 lopoda. Its gills or lungs are four instead of two ; 

 it has but a single heart ; the head is furnished with 



* See fig. 2, p. 217. 



t Mag. Nat. Hist., N.S., vol. iii. p 521. 



w, nautilos, a sailor ; its Greek name. 



