water tube

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) 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.
• Any tube for passing or holding water; specif., in some steam boilers, a tube in which water circulates and steam is generated.