\Water tube\
Any tube for passing or holding water; specif., in some steam
boilers, a tube in which water circulates and steam is
generated.

\Wa"ter tube`\ (Zo["o]l.)
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}.