70 S- GOTO. 



minutely described by Pintner'^ and Fraipont.'^ 



The wall of the excretory vessels, the larger as well as the smaller, 

 is constituted alike in all the species studied by me ; and consists of a 

 structureless, refractive membrane which is, in most species, very thin, 

 but on which a double contom* can be distinctly recognized. In. Mono- 

 cotyle, the wall is much thicker, as may be seen in fig. 5, PI. XVIII. 

 The membrane stains more or less with haematoxylin in most 

 species, and this is especially the case in Monocotijle. The wall of the 

 terminal sac is exactly of the same appearance as that of the vessels. 

 In some sections of Tristomum sinuatum, I have observed an elongated 

 nucleus lying closely appressed to the wall of the excretory capillaries ; 

 but owing to the exti'eme rarity of such observations, I could not 

 decide whether it belonged to the wall or merely to the mesenchyma. 



The calibres of the vessels are observed to vary a great deal 

 according to the various degrees of distention in which they are held 

 by the contained fluid ; and it is a necessary consequence of this that 

 the thickness of the membrane that constitutes the wall should vary 

 accordingly. 



In some sections, I have observed in the cavity of the excretory 

 vessels a flaky substance lightly stained by haematoxylin ; but this 

 was rather seldom, and in most cases the cavity was wholly empty 

 and clear. 



In my paper on Diplozoon^^ I have specified three alternative 

 ways in which the wall of the excretory vessels can be supposed to have 

 arisen, viz., either (1) it arose by a simple transformation of the pro- 

 toplasmic wall such as we find in the turbellarians, or (2) it had been 



1). Pintner— Untersuch. u. d. Bau des Bandwurmkorpers. Wiener Arbeiten. Bd. Ill, 1880. 



2). Fraipont— Eeoherolies sur I'appareil excrefcenr des Trematodes et des Cestoides. 

 Archives d. Biologie. T. I, 1880. p. 415. 



3). I. c. p. 176. 



