176 



s. GOTO. 



sometime under the cover-glass, and which are executed in such a 

 way as to drive the contained fluid towards the excretory pores. These 

 movements are probahly due to the presence of vil)rati]e flaps in the 

 wall, but I have not been able to observe them in sections. The 

 wall is seen, in section, to be f(jrmed by a compact refractive mem- 

 brane with douljle contour, which does not stain with haematoxylin 

 (Fig. 16, as). Evidences have been advanced by Lang^* and Ijinia'^ 

 that the excretory vessels of the Turbellarians are " nichts Anderes als 

 durchbohrte Zellen." In the Cestodes, Pintner^' has observed a well- 

 developed epithelium on the wall of the main vessels, "das zweifels- 

 oline als Matrix ihrer glashellen, homogenen Membran aufzufassen 

 ist." According to Schw^arze*\ the central excretory vessel of the 

 Distomes is ut first a solid string of cells, which afterward acquires a 

 lumen. He also supposes that the finer branches originate in the same 

 way, and that the structureless condition of their walls in the adult 

 worm may be explained by supposing " dass nach der Resorption des 

 Inhaltes der primären Zellen keine äussere, muskulöse Zellenlage 

 gel)ildet wird, somlerti die Wandung sicli, al lein, aus der äusseren ZeUmein- 

 hraiini der ursprüngliclieu Aidagc zusaunneiisctztr Whether the walls of 

 the excretory vessels oi Diplozoot is to be regarded as similar to those 

 of the Turbellarians, with the difference that the protoplasmic remnants 

 of the original cells have been transformed into a structureless mem- 

 brane, or whether they had been produced by a distinct epithelium 

 which afterwards underwent degeneration and finally disappeared, or 

 whether they were formed by such a process as Schwarze^- supposes 



1) Lang— 1. c. p. 212. 



2) Ijima — Untersuch, ü. d. Bau u. d. Entwickl. d. Siisswasser-Dendrocoelea. Ztschr. f. w. 

 Zool. Ed. XL. 1884. p. 397. 



3) Pintnei— 1. c. p. 21. 



4) Schwarze — 1. c. p. 58. The italics are mine. 



5) In the extreme case, viz. wliere the cells are arranged in a si agio row, the view of 

 Schwarze reduces itself to that of Lang and Ijima. 



