ON TEMNOCEPHALA. 29] 



Excretory System. 



The excretory system of Temnocephala differs from that 

 of most Trematodes 1 in opening on the exterior by two aper- 

 tures. These are situated on the dorsal surface slightly behind 

 the eyes, as already described. The surface in this position is 

 usually found to be elevated on each side into a rounded, 

 eminence, and in the centre of this is easily to be distinguished 

 the minute circular opening. This leads into the cavity of a 

 non-ciliated sac (Plate XXI, figs. 9 and 10) with thick walls, 

 of a pyriform shape, but slightly bent on itself. From the 

 narrower posterior end proceed two large vessels which pass 

 backwards along the lateral edge of the alimentary canal, and 

 two (or a single one which speedily bifurcates) passing forwards 

 towards the bases of the tentacles. As ordinarily seen these 

 canals are twisted into spirals owing to the contraction of the 

 whole body, but when the body is fully extended they are straight- 

 ened out. The arrangement and final destination of these 

 vessels and their branches I did not succeed in tracing. It 

 would seem probable that the branches open directly into the 

 spaces and channels in the parenchyma, and the " Schwingende 

 Lappchen " found in other forms do not seem to be present. 2 



The above-described arrangement of the excretory system 

 would seem to be very characteristic. Paired external open- 

 ings, where they occur in other Trematodes, are to be found on 

 the ventral and not on the dorsal surface, with the single 

 doubtful exception of Polystomum. 



The wall of the terminal sac (fig. 10) is composed of a thick 

 layer of very finely fibrous substance (<?.) without a trace of 

 nuclei, but with occasional rounded vacuoles, and a thin layer 

 of a similar substance lines the longitudinal vessels (fig. 11). 

 External to this protoplasmic layer is a thin sheet of the 

 parenchyma muscle (fig. 10, m.) which completely invests the 



1 Vide Fraipont's <c Recherclies sur l'appareil excreteur des Trematodes et 

 des Cestodes," ' Archives de Biologie," ii (1881). 



2 Taschenberg states that they are absent also in Tristomum (see ' Zoolo- 

 gischer Jahresbericht,' 1879, i, p. 319). 



