1. 
   Water tube
   Wa'ter tube` (Zoology) 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 .
   Found op http://www.encyclo.co.uk/webster/W/15
   
   
2. 
   Water tube
   Water tube Any tube for passing or holding water; specif., in some steam boilers, a tube in which water circulates and steam is generated.
   Found op http://www.encyclo.co.uk/webster/W/15
   
   
3. 
   water tube
   <zoology> 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. ... Source: Websters Dictionary ... (01 Mar 1998) ...
   Found op http://www.mondofacto.com/facts/dictionary?water+tube
   
   
4. 
   Water tube
   • Any tube for passing or holding water; specif., in some steam boilers, a tube in which water circulates and steam is generated. • 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 kidney...
   Found op http://thinkexist.com/dictionary/meaning/water_tube/
   
   

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