\Nau"ti*lus\, n.; pl. E. {Nautiluses}, L. {Nautili}.
[L., fr. gr. nayti`los a seaman, sailor, a kind of shellfish
which was supposed to be furnished with a membrane which
served as a sail; fr. nay^s ship. See {Nave} of a church.]
1. (Zo["o]l.) The only existing genus of tetrabranchiate
   cephalopods. About four species are found living in the
   tropical Pacific, but many other species are found fossil.
   The shell is spiral, symmetrical, and chambered, or
   divided into several cavities by simple curved partitions,
   which are traversed and connected together by a continuous
   and nearly central tube or siphuncle. See
   {Tetrabranchiata}.
Note: The head of the animal bears numerous simple tapered
      arms, or tentacles, arranged in groups, but not
      furnished with suckers. The siphon, unlike, that of
      ordinary cephalopods, is not a closed tube, and is not
      used as a locomotive organ, but merely serves to
      conduct water to and from the gill cavity, which
      contains two pairs of gills. The animal occupies only
      the outer chamber of the shell; the others are filled
      with gas. It creeps over the bottom of the sea, not
      coming to the surface to swim or sail, as was formerly
      imagined.
2. The argonaut; -- also called {paper nautilus}. See
   {Argonauta}, and {Paper nautilus}, under {Paper}.
3. A variety of diving bell, the lateral as well as vertical
   motions of which are controlled, by the occupants.