ON THE NEPHRIDIA OP THE POLYCH^TA. 441 



organ, into which opens the " ciliated organ." The outer lip of 

 the latter stretches out to the body-wall. The true nephridium is 

 spread over the surface of the nepliridial sac, and has no internal 

 opening (cf. figs. 1, 3, and 30). The description applies to 

 both sexes. 



The Nephridium. — The structure of the nephridia in the 

 three species of Glycera I have studied differs only in detail. 

 The nephridium opens to the exterior ventrally by a minute 

 pore, situated just outside the limit of the large bundles of 

 ventral longitudinal muscles. The nephridiopore leads into a 

 very slender canal, difficult to follow in sections, which passes 

 through the body- wall into the septum above ; then running 

 inwards and piercing the septum, the canal reaches its anterior 

 face, where it soon joins the nephridial sac. On arriving here 

 the canal divides repeatedly, giving off branches which spread 

 over the outer surface of the sac. 



In Glycera convolutus, where the nephridium is small 

 and the sac scarcely developed (figs. 13 and 21), the body of 

 the nephridium forms a somewhat flattened pear-shaped mass 

 applied to the ciliated organ. A fresh nephridium dissected 

 out and examined in sea water appears to consist of a proto- 

 plasmic mass in which the lumen of the canal branches, form- 

 ing a sort of sponge- work. Along the course of the canals 

 are rounded diverticula or chambers, projecting towards the 

 free coelomic surface of the organ (figs. 13 and 12). 



The wall of the lumen of the canal leading to the exterior, 

 and of its main branches, is provided with long cilia (fig. 12) ; 

 the smaller branches leading from the chambers are also 

 ciliated, but to a less extent. By their action the cilia tend to 

 drive a current from the chambers towards the external pore. 



The protoplasmic walls of the nephridium are very granular, 

 being more or less loaded with excretory matter in the form of 

 granules or droplets, small and large, which often give the 

 organ a yellowish colour. There are nuclei here and there, 

 but I have seen no distinct cell outlines. 



The outer wall or roof of the chambers projecting towards 

 the ccelom is very thin, and arising from near the centre of 



VOL. 41, PART 3. — NEW SERIES. H H 



