The Collaborative International Dictionary of English v.0.48:

Tube \Tube\, n. [L. tubus; akin to tuba a trumpet: cf F. tube.]
   1. A hollow cylinder, of any material, used for the
      conveyance of fluids, and for various other purposes; a
      pipe.
      [1913 Webster]

   2. A telescope. "Glazed optic tube." --Milton.
      [1913 Webster]

   3. A vessel in animal bodies or plants, which conveys a fluid
      or other substance.
      [1913 Webster]

   4. (Bot.) The narrow, hollow part of a gamopetalous corolla.
      [1913 Webster]

   5. (Gun.) A priming tube, or friction primer. See under
      Priming, and Friction.
      [1913 Webster]

   6. (Steam Boilers) A small pipe forming part of the boiler,
      containing water and surrounded by flame or hot gases, or
      else surrounded by water and forming a flue for the gases
      to pass through.
      [1913 Webster]

   7. (Zool.)
      (a) A more or less cylindrical, and often spiral, case
          secreted or constructed by many annelids, crustaceans,
          insects, and other animals, for protection or
          concealment. See Illust. of Tubeworm.
      (b) One of the siphons of a bivalve mollusk.
          [1913 Webster]

   8. (Elec. Railways) A tunnel for a tube railway; also
      (Colloq.), a tube railway; a subway. [Chiefly Eng.]

   Note: In the New York area, the subways running under the
         Hudson River are sometimes referred to as the tube.
         [Webster 1913 Suppl. +PJC]

   Capillary tube, a tube of very fine bore. See Capillary.


   Fire tube (Steam Boilers), a tube which forms a flue.

   Tube coral. (Zool.) Same as Tubipore.

   Tube foot (Zool.), one of the ambulacral suckers of an
      echinoderm.

   Tube plate, or Tube sheet (Steam Boilers), a flue plate.
      See under Flue.

   Tube pouch (Mil.), a pouch containing priming tubes.

   Tube spinner (Zool.), any one of various species of spiders
      that construct tubelike webs. They belong to Tegenaria,
      Agelena, and allied genera.

   Water tube (Steam Boilers), a tube containing water and
      surrounded by flame or hot gases.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water tube \Wa"ter tube`\ (w[add]"t[~e]r t[=u]b`).
   1. (Zool.) One of a system of tubular excretory organs having
      external openings, found in many invertebrates. They are
      believed to be analogous in function to the kidneys of
      vertebrates. See Illust. under Trematodea, and
      Sporocyst.
      [1913 Webster]

   2. Any tube for passing or holding water; specif., in some
      steam boilers, a tube in which water circulates and steam
      is generated.
      [Webster 1913 Suppl.]