RENAL ORGANS.—The kidneys consist of a pair of convoluted tubes, formed of a succession of perforated, so-called "drainpipe" cells (Fig. 106, _k_); they open directly or indirectly into the cloaca. Their walls are thin in the straight parts, but thick and glandular in the coils which occur at intervals. These tubes bear little tag-like appendages, hanging freely into the body-cavity, often widening towards the free end, and flattened or circular in section (Fig. 106, _ns_). They show during life a peculiar flickering motion in their interior, like the equivalent "flame-cells" of many Platyhelminthes (see p. 25), and are in function the representatives of the multicellular renal funnels of Annelids. On one side, especially on the edge of the flattened tags, the appearance is as of a tapering whip-like lash, attached by its base to the free end of the tag and waving in its cavity; but the side view of the flattened tags shows an appearance of successive transverse or oblique waves. In many if not all cases the free end of the tag is closed by a vacuolated plug of protoplasm, which sometimes at least bears two flagella waving freely in the body-cavity. The probable explanation of the two distinct wave appearances within the tag is that the protoplasmic plug bears on its inner face a row or tuft of long cilia hanging down into the cavity of the tag. The tags probably keep up a current of liquid through the kidneys, while the contents of the body-cavity are constantly replenished by osmosis. The two renal tubes may end blindly below the disc, or else join by a short transverse dorsal communication in front of the brain, as in _Stephanoceros_, _Atrochus_ (Fig. 112, C), and _Apsilus_ among Flosculariaceae, _Lacinularia_ among Melicertidae, and _Hydatina_ among the Illoricate Ploima (Fig. 106, _rc_). In some species of _Asplanchna_, if not all, a recurrent branch occurs opening at either end into the main tube of its own side. The kidneys unite to discharge into the cloaca near its orifice, and on its distal (primitively ventral) side in many Melicertidae. In Bdelloida the common duct formed by their fusion opens into the ventral side of a dilated bladder-like section of the cloaca (Fig. 109, A, _bl_), which contracts rhythmically to discharge the liquid; while in the majority of the class they open singly or by a common duct into a separate contractile vesicle or _bladder_, which also discharges at regular intervals into the cloaca on its ventral or distal side (Figs. 106, _bl_ and 112).