No. I.] GERAf -LAYERS IN CLEPSTNE. jgjr 



entstehen einerseits die Mesenterien, anderseits die Dissepi- 

 mente." 



In harmony herewith, it is inferred that the permanent neph- 

 ridia served primarily as ducts for the escape of the sexual 

 products, the excretory function having developed later. This 

 view may appear plausible enough so long as we assume with 

 Bergh that the larval and the permanent nephridia represent two 

 unrelated systems of organs, having no connection with each 

 other either ontogenetically or phylogenetically. But, let their 

 homogeny be conceded, and there will be no escape from the 

 conclusion that the functional relations of the permanent 

 nephridia to the sexual organs, wherever such relations exist, 

 have been acquired secondarily. The question then becomes 

 simplified ; for we have only to determine the primitive function 

 of the provisional nephridia. As these organs never function 

 as sexual ducts we have no reason to suppose that they have 

 ever served any other purpose than that of excretory organs. 

 As the permanent nephridia arose later, either directly from the 

 larval organs, or, at least, from the same basis ; as they exhibit 

 the same general structural features ; and as their appearance is 

 followed by the atrophy of the larval system, there is every reason 

 to believe that they assumed the work of the organs which 

 they superseded. It is easy to understand how such organs 

 could be pressed into the service of the sexual organs, and how 

 their original function might be suppressed as the result of 

 adaptation to this new work. The conversion of sexual ducts 

 into excretory ones, presenting the typical structure of the 

 primordial excretory organs, could not, on the other hand, be 

 so readily explained. 



Original Basis. — The question of original basis, like that 

 of original function, must be considered in the light of what is 

 known about the development of the larval nephridia. In the 

 leeches these organs appear to arise from single cells, which 

 develop, by division, into simple cell-cords. This simple 

 mode of development is repeated in the ontogeny of the meta- 

 meric nephridia, as seen in the formation of nephridial cell- 

 rows from terminal nephroblasts. Although the nephric cell- 

 plates, into which the primary cell-cords are metamerically 

 divided, consist of numerous cells, it is probable that each 

 plate represents a simple (or double) string of cells, with its 



