SEGMENTAL ORGAN OF PODARKE OBSCURA. 287 



Podarke obscnra has been shown to possess a simple, uncoiled 

 nephridial tube with a well differentiated ciliated organ develop- 

 ing at the same time and probably in union with the nephridium. 

 No trace of a phagocyte organ was observed. 



As a result of the investigations in this one family of the 

 Polychaetes, we may divide the Hesionidae into three groups. 



This classification is modeled somewhat after that made by 

 Goodrich for the whole group of Polychaeta. The nephridium 

 has always an internal opening. Solenocytes have not been 

 observed. 



Group 1. — Forms in which the segmental organs consists of 

 a nephridium highly differentiated, having a coiled tube and a well- 

 developed nephrostome, and a distinct independent ciliated organ 

 or genital funnel with an external opening through which the eggs 

 are discharged. Hesione pantherina. 



Group 2. — Forms in which the nephridium is a more simple 

 tube opening into the ccelome by a small nephrostome. At the 

 time of reproduction a ciliated organ is grafted on to the tube and 

 affords a means of exit for the genital products. Ophiodromus 

 flexuosus, Oxydromus propinquus, Kefersteinia cirrata. 



Group j. — Forms in which the nephridium is still more simple, 

 having no coils. The nephrostome has been finally lost and its 

 place taken by the ciliated organ or genital funnel, which develops 

 at the same time, and probably in union with the nephridium. 

 In this one group we may find forms illustrating the gradual loss 

 of the nephrostome and the final fusion of the ciliated organ. 

 Hesione sicula, Tyrrhena, Irma, Podarke obscura. 



In this classification we have a gradual degeneration or modi- 

 fication of the nephridium from a condition where it performs its 

 excretory function only, through a condition where it has become 

 adapted temporarily to the secondary function of offering a means 

 of sexual discharge to a condition finally where it has become so 

 modified that from an early stage, if not throughout life, it func- 

 tions as an excretory duct and a genital tube. 



Columbia University, 

 April, 1907. 



