660 DAVID HILT ''TENNENT. 
The wall of the sac is thin, formed of a single layer of 
greatly flattened cells whose nuclei bulge into the cavity 
(fig. 37). 
In fresh unstained specimens little more than this sac may 
be seen. In living specimens which are slightly stained with 
methylene blue, the longitudinal trunks of the water vascular 
system may be seen. Their arrangement is the same as that 
described by Ziegler for B. polymorphus. From about the 
middle of each side of the bladder a short duct is given off, 
which divides almost immediately into an anterior and a pos- 
terior lateral trunk. These trunks extend forwards and back- 
wards respectively, receiving, from time to time, branches at 
whose extremities lie flame-cells (fig. 59). 
The flame-eells are seen with difficulty. I was able to ob- 
serve them mostreadily when the slightly stained Bucephalus 
had remained under a coverslip for about twenty minutes. 
In such preparations they might be watched for but a short 
time, their activity ceasing within a few minutes. 
The excretory pore on the ventral surface may be seen in 
the living animal. Sections reveal a somewhat strange 
arrangement of outlets. From the duct passing from the 
contractile sac to the cavity in the tail, two short canals are 
given off, one of which passes to the dorsal surface, where it 
opens through a very fine pore, and one to the ventral surface 
at the junction of the body with the median portion of the 
tail (figs. 24 and 25). 
When I first noticed this peculiar condition I attributed it 
to some abnormality, but an examination of several series 
convinced me that it is the usual arrangement. 
A layer of sphincter muscles surrounds the outlet of the 
sac and prevents the return of liquid forced into the tail. 
Tae GENITAL ORGANS. 
In Bucephalus the reproductive organs are rudimentary. 
By comparison with the adult stage, Gasterostomum, we are 
able to identify an elongated mass of nuclei which stain deeply 
