No. 3-] THE DISCODRILID NEPHRIDIUM. 34 1 



The simple tubules (Fig. i, ct, and Fig. 39) which alternate 

 with and connect the plexus nodules are short lengths of "drain- 

 pipe " cells having a diameter of .03 mm. at the middles of 

 their lengths, and slightly more at their ends, where they enlarge 

 to pass into the nodules. They may be straight, or more or 

 less curved, according to local conditions (Fig. i). The lumen 

 by which each is perforated is a simple canal of about .013 mm. 

 diameter, but enlarging more or less at the ends to pass into 

 the plexuses. The walls of the tubules are more transparent 

 than the nodules, but granular and radially striated (Figs. 16, 



39)- 



Each tubule contains a conspicuous bunch of cilia. These 

 are few in number, and are arranged on one side of the lumen, 

 arising usually from its efferent end and nearly opposite to the 

 nucleus. They seem to be arranged in a longitudinal series 

 (Fig. 39), but in a few cases appear to be attached to a trans- 

 verse ridge. Though few in number, the cilia are very long 

 (.05 mm.), and commonly reach the entire length of the tubule; 

 sometimes in short ones their free ends extend into the begin- 

 ning of the following plexus (Fig. i). Beating with a move- 

 ment that is at once undulating and rotary, and so rapidly that 

 I was unable to satisfactorily count the movements (about 140 

 per minute at 50° F.), these powerful cilia must exert a con- 

 siderable force in propelling the fluids onward. Their free ends 

 frequently become caught on the sides of the tubules, and more 

 or less obscured in a mass of granules which collects at such 

 points. In other cases the cilia become adherent side by side 

 in collected mucous and granules, and act as an undulating 

 membrane. Cilia are never found in the plexuses except in the 

 last one or two of the accessory lobe nodules, into the ex- 

 cavated chambers of which they may extend as a few isolated 

 patches of shorter cilia. The mechanical advantage and, 

 indeed, necessity of the cilia in the narrow passages are evident, 

 the cross section of these being many times less than the 

 aggregated cross sections of the plexus passages, and the velocity 

 of the stream of passing fluid as many times as great. 



Each of the short tubules contains a prominent nucleus lying 

 in the wall close to the lumen, usually near its middle, but 



