428 
W. A, HASWELL. 
this group, similar structures occurring in some of the 
Heterocotylean Trematodes (Braun, 2 ; Groto, 4). But the 
nature of these sacs and their relations to the system of 
vessels in Temnocephala are quite unlike anything that is 
known to occur in other forms. 
The most essential features of this system were described 
by me in 1893 (8). But since some of the most important of 
these points have been overlooked by recent writers,^ or 
their significance not recognised, it seems desirable to give a 
brief resume of the facts. 
Each of the two dorsally situated excretory apertures leads 
into a thick-walled contractile terminal vesicle (PI. 25, 
fig. 18), which is of pyriform shape, bent on itself towards 
its apex. The contractions of the walls of the vesicle are 
effected by the agency of an enclosing’ layer of muscular 
fibres, and the external aperture is surrounded by a mus- 
cular sphincter. Between the muscular layer and the proper 
wall of the vesicle is a layer of loose parenchyma, which 
doubtless facilitates freedom of movement. The vesicle 
itself consists of two large cells fused together and hollowed 
out to form the lumen. The greater part of the wall of the 
vesicle is formed by one of these two cells ; the narrower 
apical part, and the beginning of the main duct which is 
given off from it by the other. The relative position of the 
two cells is indicated mainly by the position of their two 
nuclei, but tlie texture of the protoplasmic substance of the 
two cells differs somewhat in character, and that of the 
smaller is much more susceptible to the action of staining 
agents. 
The inner surface of the vesicle is quite smooth and uni- 
form. Where an occasional exception to this appears to 
occur, and the usually sharp internal outline appears blurred, 
* Plate (15) and Wacke (16) for example. The former states, “An 
den Nepliridien der Temnocephaliden ist bis jetzt vergeblicli nacb den 
fiir die Platybelmintben so characteristiscben Flimmerzellen gesucbt 
worden.” Yet I bad described the system with its flame-cells in a 
paper in the ‘ Zoologiscben Anzeiger’ two yeai-s before (1892). 
