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.  [1913 Webster]

 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.]