KIDNEYS 213 



some species of Albertia, which cling to their host by the 

 exserted trophi. 



Renal Organs. The kidneys consist of a pair of convoluted 

 tubes, formed of a succession of perforated, so-called "drain- 

 pipe" cells (Fig. 106, A:); 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 proto- 

 plasm, which sometimes at least bears two flagella waving freely 

 111 the body-cavity. The probable explanation of the two dis- 

 tinct 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, Atrockus (Fig. 112, C), and Apsilus 

 among Flosculariaceae,Zmtz</ri among Melicertidae, and Hijda- 

 tinfi among the Illoricate Ploima (Fig. 106, ?r). 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 Meli- 

 certidae. 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, W), which contracts rhythmically to dis- 

 charge the liquid ; while in the majority of the class they open 

 singly or by a common duct into a separate contractile vesicle or 



