538 
MoUusca. 
THE NAUTILUS, OK PEARLY NAUTILUS, 
(Nautilus Pompilim,) 
Is a very different creature, and instead of the eight 
arms of the Argonaut has its head surrounded by 
numerous ringed and sheathed tentacles. It is remark- 
able for the structure of its shell, the cavity of which is 
divided into numerous chambers by transverse parti- 
tions : these chambers, of which the outermost alone is 
occupied by the animal, are filled with air, but a narrow 
tube passes through the whole of them, and communi- 
cates with the cavity of the body. By this arrangement 
the Nautilus is enabled to alter his specific gravity so as 
either to rise to the surface or sink to the bottom of the 
water. The few existing species of Nautilus are all 
found in the Indian and South Pacific Oceans. 
