284 ^^^- tVoL. VII. 



Both Graff and Landsberg state that the smaller branching 

 limb of the tube lies ventral to the larger limb. 



Careful focusing upon the living worm which has been com- 

 pressed under a cover glass shows that the fine branches of the 

 smaller limb and the limb itself lie dorsal to the larger limb. 

 In a favorable worm which is compressed these fine branches 

 may be traced not only to the middle of the body, but Jo the 

 posterior end. 



From the fact that excretory cells (" Wimpertrichtern ") have 

 been described in connection with the terminal branches of the 

 water vascular tube in other Rhabdocoels {Mesostovia, Plagios- 

 toma, and Derostonid) there can be little doubt of their existence 

 in Stenostonia. I have been unable to find these cells in sec- 

 tions and have tried numerous anilines, with a hope of finding 

 a specific stain that would show them in the living worm, but 

 always without success. As previously stated, methylin blue 

 stains the pharyngeal cells without affecting the cells of the 

 other tissues. 



It has not been found possible to show by direct observation 

 that the pharyngeal cells are connected with the terminal 

 branches of the water vascular system, or that they are not so 

 connected. If they are so connected, they are the specific ex- 

 cretory cells as claimed by Zacharias. They cannot be the only 

 specific excretory cells, since we must suppose that every ulti- 

 mate branch of the water vascular tube terminates in such a 

 cell ; and these ultimate branches are much more numerous in 

 the region posterior to the pharynx than in the region over the 

 pharynx. 



There are, therefore, but two alternatives open to us. We 

 must assume, either (i) that there are two kinds of specific ex- 

 cretory cells in Stenostoma leiicops, one of which (the pharyn- 

 geal cells) is readily stained in the living worm by methylin 

 blue, and the other of which remains unstained ; or (2) that the 

 pharyngeal cells are not excretory in the sense of being con- 

 nected with the water vascular system, and that the true excre- 

 tory cells of the water vascular system have not yet been 

 demonstrated. All the observations which have been made 

 seem to me to be opposed to the first assumption. 



