4.06 WILLIAM NICOLL. 
T'rematode parasites, do not appear to suffer from their 
presence, but may indeed possibly benefit by the increased 
stimulation of the gastric and intestinal glands. An alternative 
theory with some claim to recognition is that the secretion of 
these glands renders the contents of the host’s intestine less 
tenacious and thereby facilitates the activity and locomotion 
of the parasite. 
The alimentary system does not offer any striking 
peculiarity. There is a small pre-pharynx, with thin walls 
and lined by cuticle continuous with that lining the oral 
sucker and pharynx. Into this the anterior end of the 
pharynx projects, so that, on external view, the pharynx 
appears to be continuous with the oral sucker. In many 
species the pharynx is described as continuous with the oral 
sucker, but in most of those it will probably be found that 
a small pre-pharynx intervenes. One notable exception is 
Rhytidodes gelatinosus (Rud.), in which no pre-pharynx 
is present. The pre-pharyngeal vestibule gives the pharynx 
oreater freedom of movement, permitting, as it does, a certain 
amount of longitudinal movement, independent of the body 
extension and contraction, while at the same time it allows 
the anterior end of the pharynx to expand, somewhat after 
the manner of a sucker. ‘The external membrane of the oral 
sucker is continued along the pre-pharynx on to the pharynx, 
which it invests. The pharynx is of moderate size, and 
nearly globular, but fattened dorso-ventrally. Its diameter 
is ‘15-19 mm. and thickness ‘(09-10 mm. The lumen is a 
narrow, transverse slit, and the walls are about ‘04-05 mm. 
thick. The structure is identical with that of the suckers. 
The radial muscle-fibres have an average thickness of 
‘0015 mm. and the circular fibres ‘(001 mm. Towards the 
anterior end the muscle-fibres are very closely packed. 
Passing forwards the radial fibres are more and more obliquely 
set, until they come to run almost parallel to the internal 
wall. Lacunar spaces are much reduced and cells are rare. 
Most movement apparently takes place at this part. 
The cesophagus is about the same length as the pharynx 
