add. ·- any tube for passing or holding water; specif., in some steam boilers, a tube in which water circulates and steam is generated.
2. water tube ·- 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.