STRUCTURE OF THE NEPHRIDIA OF DINOPHILUS. 531 



dorsal pores of Polyclads, and also the openings of the larval 

 excretory organs of the Actinotrocha larva, it may be claimed 

 justly that I have adduced, perhaps, no conclusive evidence 

 for their absence in Din op hi Ins, to this I can only say that 

 after prolonged investigation of this point I can find no evi- 

 dent traces of their presence. It is, perhaps, worthy of note 

 that Schimkewitsch (23) in the White Sea species also can 

 find no evidence of external openings to the nephridia by the 

 methylene-blue method of impregnation. 



In several preparations the canals could be traced down to 

 these vacuoles which appeared closed, but careful examination 

 of the external surface over the vacuole seemed to show the 

 presence of a slight depression. In this depression a number 

 of granules were adherent to the external surface, these 

 moved backwards and forwards as if disturbed by the escape 

 of fluid from the interior of the vacuole. Frequently they 

 were noticed to have a peculiar dancing movement. I could 

 never see distinct evidence, however, of the escape of any 

 fluid, and the granules were themselves never seen to be de- 

 tached or actually washed from their places as one might 

 suppose would take place if any fluid was being discharged 

 from the vacuole. Close examination of the surface of the 

 vacuole when the movement of the granules was most evident 

 failed also to reveal traces of any external pore. In some 

 cases the granules seemed to dance round one particular point 

 in the middle of the surface of the vacuole, but close examina- 

 tion of this under a Zeiss 2 mm. oil-immersion objective com- 

 bined with 12 m. and 18 ra. compensating ocular, with careful 

 lighting, failed to show the presence of any opening. I am 

 inclined to think the fluid escapes from the vacuole by osmo- 

 sis through its wall, and this escape of fluid accounts for the 

 movement of the granules which themselves may be of an 

 excretory nature. They are frequently orange coloured like 

 the granules of the blastocoelic space, though very much 

 smaller, and are probably deposited on the external surface 

 from the slowly excreted fluid of the vacuole as it reaches the 

 exterior. 



