ON THE NEPHRIDIA OF THE POLYCHiETA. 443 



there is no communication of the lumen of the ne- 

 phridium with the coelom, either directly or indi- 

 rectly, through the nephridial sac, in this or any other species 

 of Glycera. 



Over the chamber-bearing surface of the actual nephridiura 

 there appears to be no regular layer of coelomic epithelium. 

 An occasional nucleus here and there may indicate its remains, 

 but the nephridium seems to have made its way through the 

 epithelium, as in the case of the nephrostome of an ordinary 

 earthworm. 



In the much larger species, Gl. sip h on os to ma, the nephri- 

 dial complex is a structure of considerable size (figs. 3 and 

 30), visible even to the naked eye. Figs. 13 and 3 represent 

 these organs in Gl. convolutus and siphonostoma respec- 

 tively, drawn to the same scale. Here the nephridial sac is 

 very much more developed, and the nephridium itself extends 

 almost all over its surface, forming a sort of outer layer or 

 shell (fig. 30). The system of branching canals is extremely 

 complicated, and the number of chambers immense. Their 

 structure is best studied in the very similarly developed species, 

 Gl. unicornis. 



The solenocytes (tube-bearing cells) in Gl. siphonostoma, 

 though so much more numerous, closely resemble those just 

 described in Gl. convolutus. Occasionally I have noticed 

 little ameeboid processes with thickened ends radiating from 

 the cells (fig. 4) ; they remind one of the pointed proto- 

 plasmic processes originating from the tube-bearing cells of 

 Nephthys (fig. 4), but appear to arise rather from the bases of 

 the cells where these are applied to each other. 



The third and last form which I shall describe, Gl. uni- 

 cornis, is intermediate in size between the first two species. 

 Here the sac is large, but the nephridium does not, as a rule, 

 cover over its whole surface, being specially developed along 

 the rim of the disc (figs. 2, 11, 16, and 26). There seems, 

 however, to be considerable variation in the extent to which 

 the nephridium spreads over the sac ; in some specimens it 

 forms a layer covering its whole surface, as in Gl. siphono- 



