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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.

he radical zones of echinoderms, along which run the principal nerves, blood vessels, and water tubes. These zones usually bear rows of locomotive suckers or tentacles, which protrude from regular pores. In star fishes they occupy the grooves along the under side of the rays.

r Chain. -- Ring blackbird (Zoöl.), the ring ousel. -- Ring canal (Zoöl.), the circular water tube which surrounds the esophagus of echinoderms. -- Ring dotterel, or Ringed dotterel. (Zoöl.) See Dotterel, and Illust. of Pressiroster. -- Ring dropper, a sharper who pretends to have found a ring (dropped by himself),…

s that construct tubelike webs. They belong to Tegenaria, Agelena, and allied genera. -- Water tube (Steam Boilers), a tube containing water and surrounded by flame or hot gases.

A pore by which the water tubes of various invertebrates open externally.