ON THE CGELOM, GENITAL DTJCTS, AND NEPHRIDIA. 479 



groups as the Mollusca, Arthropoda, Ectoprocta, Echinoderma, 

 and Vertebrata, there are in the adult no certain traces of true 

 nephridia. In these latter groups, as we shall see, the peri- 

 toneal funnels (primitive genital ducts) take on the excretory 

 functions of the nephridia which they supersede. 



In the following brief review of the various classes of Coelo- 

 mata, I shall endeavour to show that the two kinds of organs 

 can always be distinguished ; that the first, the nephridium, is 

 primitively excretory in function, is developed centripetally 

 as it were, and quite independently of the coelom (indeed, is 

 probably derived from the epiblast), possesses a lumen which 

 is developed as the hollowing out of the nephridial cells, and 

 is generally of an intracellular character, is closed within, and 

 may secondarily acquire an internal opening either into a blood 

 space or into the coelom (true nephridial funnel as opposed to 

 the peritoneal funnel) ; and that the second kind of organ, the 

 peritoneal funnel, is primitively the outlet for the genital pro- 

 ducts, is invariably developed centrifugally as an outgrowth 

 from the coelomic epithelium or wall of the genital follicle, is 

 therefore of undoubtedly mesoblastic origin, and possesses a 

 lumen arising as an extension of the coelom itself. 



In the series of diagrams illustrating this paper, based on 

 the most recent and accurate researches, it has been my con- 

 stant endeavour to interpret the author's results correctly, and 

 not to distort the facts in favour of the theory here advocated. 



Planarians. 



The nephridia of the Planarians, as is well known, are 

 formed of a main duct, which branches out into fine tubules 

 ending blindly internally in flame-cells (fig. 1); they do not 

 develop beyond this " pronephridial " condition — protonephri- 

 dium of Hatschek (65). The arrangement of this single pair 

 of nephridia is extremely variable j the two organs may join 

 and open by a median external pore near the mouth or behind, 

 or they may open by a number of pores at the sides. Gun da 

 segmentata, a most interesting form described by Professor 

 Lang (69), possesses longitudinal main trunks into which 



