STRUCTURE OF THE NEPHRIDU OP DINOPHILUS. 529 



canals. Schimkewitsch (23) shows these in some of his trans- 

 verse sections. I have, however, never been successful in 

 seeing them in sections. In sectious like those shown in 

 figs. 12 — 15, taken from material treated with cocaine aud 

 carefully fixed in Hermaun's fluid, and afterwards treated 

 with Zenker's fluid, no trace of the nephridial canals could 

 be found. 



II. The First Nephridium. 



The position and general course of the canal of the first 

 nephridium has been very correctly given by Harmer (8), 

 indicated in his figure (PI. 10, fig. 15). The main portion of 

 the nephridium lies in a plane slightly dorsal to the wall of 

 the pharynx when the animal is flattened slightly. The 

 testis extending into the head region is pierced by the canal 

 which terminates ventrally to it, but probably some distance 

 lateral to the median line. Throughout its course it is 

 seldom seen to undergo any variation in size during the 

 movements and contractions of the worm, Avhich suggests 

 that its walls are composed of some fairly firm substance, 

 which does not allow of the canal being readily compressed 

 and the lumen of the canal obliterated. In none of the 

 nephridia can any trace of an actual opening of the canal on 

 the surface be seen, and in all cases they would appear to 

 terminate at the basement membrane of the epidermis. The 

 closest examination fails to reveal any traces of an external 

 pore. The point at which the canal terminates, and up to 

 which the flagella in its interior can be traced, is but a very 

 short distance from the surface, careful measurement show- 

 ing it to be less than "004 of a mm., yet the external surface 

 is perfectly intact. The terminal point of the nephridium is 

 well shown in fig. 7, which represents a portion of the margin 

 of a preparation under high magnification. The canal is 

 plainly seen to end at the limiting membrane of the epidermis 

 opposite a large vacuole into which sometimes the ends of the 

 flagella are seen beating, not, however, under normal condi- 



