970 



TnOS. H. MONTGOMERY, 



made, to show whether in the Neraerteans , as in the Annelids , the 

 excretory ducts have a different origin than the main ducts. 



C. Conclusions. 

 The nephridium of Stichostemma eühardi diff'ers from that of all 

 other Nemerteans, as far as known, in the following points: 



1) Instead of a single pair of nephridia, there are a number of 

 consecutive ones on each side of the body. 



2) Not all the nephridia are provided with excretory ducts. 



3) The nephridia extend from the anterior to the posterior end 

 of the body. 



4) The large number of excretory ducts (only exceeded by Va- 

 lencinia, cf. Bürgek, '95). 



5) The terminal bulbs have a closed cavity, not in open com- 

 munication with the lumen of the ductules. 



6) The presence of a closed cuticular structure immediately sur- 

 rounding the cavity of the bulb , produced probably by the cells of 

 the latter. 



7) The probable absence of a ciliary flame within the cavity of 

 the bulb. 



8) The comparatively great length of the ductule connecting the 

 bulb with the main ducts, and the absence of nuclei in its wall. 



9) The presence of a cuticula, of considerable thickness, on the 

 epithelium of the main ducts. 



The explanation of these remarkable diff'erences can probably be 

 given only by the adaptation of this descendant of marine ancestors, 

 to a life in fresh water. For certainly the occurrence of numerous 

 separate nephridia is not the primitive condition in this group of 

 animals, but is a secondary formation. Two suppositions as to the 

 structure of these organs in the parent form of Stichostemma, would 

 suggest themselves: either the nephridia were originally present in 

 the form of a single, short pair, such as we find in the marine Tetra- 

 stemma and Amphiporus; or in the ancestral species they existed as 

 a pair of elongated nephridia, such as we find in Eunemertes, Nemert- 

 opsis, Frosorhochmus hisiriatus (Bürger, '92, '95), and probably in 

 Tetrastemma obscurum G. (M. Schultze, '51). In the former case, 

 the course of development leading to the structural relations in Sticho- 

 stemma, would have consisted in the addition of accessory nephridia 

 behind the first, primitive pair; in the second case, by the splitting 

 up of the originally single pair, into a number of pairs. But which 



