LIFE-HISTORY OF BUCEPHALUS HAIMEANUS. 659 
It is interesting to note that the large cells composing this 
organ have an appearance similar to that of the gland cells 
of the body, and that their contents have a like staining 
reaction. 
Tue Digestive TRACT. 
At about the middle of the ventral side of the body there 
is a slight depression, at the bottom of which lies the mouth 
(fig. 41). This passes immediately into the pharynx, which 
is a thick-walled muscular organ with an outer and an inner 
set of circular muscles, an outer and an inner set of longi- 
tudinal muscles, and many radial muscles extending from 
the outer to the inner surface of the organ. 
The short cesophagus passes obliquely forward from the 
pharynx to the gut. The wall of the gut consists of a single 
layer of cells. Its shape in younger stages is extremely vari- 
able. It may be simple, i.e. a rounded sac, or its walls may 
be pushed out into numerous cecas. Ultimately the structure 
becomes rounded up, and in the oldest cercariz is seen only 
as a subspherical cavity. 
Lacaze-Duthiers (20) mentions a yellowish colour in the 
gut which I have never been able to observe. 
So far as I have observed the gut is empty at this stage. 
It is probably not functional until later. 
Tae Water VASCULAR SYSTEM. 
On examination with a low magnification the somewhat S- 
shaped contractile sac of the water vascular system may usu- 
ally be seen without difficulty. Upon watching it for a time 
it may be seen to contract quickly, when its contents are 
forced back into the middle piece of the tail, and thence into 
the lumen of the tails. The contraction takes place rhythmic- 
ally, and at every contraction of the water vascular sac there 
is an almost simultaneous expansion of the membranous por- 
tion of the tail. 
