STUDIES ON THE DIGENETIC TREMATODES. 401 
the limits in size at 2-4 mm., but Olsson found examples 
measuring as much as 6 mm. ‘The greatest breadth occurs at, 
or just behind, the ventral sucker, and is approximately one 
quarter of the length. ‘This proportion is fairly constant. 
The suckers are well developed and muscular. The 
oral sucker is sub-terminal and round, but not quite 
globular. In a specimen of 5°5 mm. length (to which all 
the following measurements may be referred) its transverse 
diameter measures ‘31 mm., while its antero-posterior diameter 
is usually slightly greater. Its aperture is not exactly 
circular, being encroached on laterally, but to no great extent, 
by the part bearing the two ventral papille. ‘he wall of 
the sucker is not of the same thickness throughout, the 
anterior dorsal part being much thicker than the remainder. 
Here the thickness is ‘(093 mm.; the rest of the wall varied 
from ‘05 mm. to ‘(065 mm. It presents a further peculiarity 
in that the thick anterior part is separated from the rest by 
a distinct constriction where the thickness is not more than 
°035—04 mm. The appearance suggests that the thick anterior 
part is a secondary growth from the main part of the sucker. 
The histological structure of the sucker represents what must 
be regarded as the typical Distomid structure. Externally 
it is separated from the body parenchyma by a well-defined 
fibrous membrane. Its cavity is lined by a cuticular layer 
continuous with the body cuticle, but somewhat thinner. 
Beneath this is a thin fibrous membrane continuous with the 
external membrane. Between the external and internal 
membranes run the radial muscle-fibres arranged in groups, 
each containing from six to twelve fibres. The latter may 
attain a diameter of ‘(0045 mm., but this naturally varies with 
the state of contraction and is usually not more than ‘002 
mm, ‘The groups are separated from each other by spaces in 
which lie one or more cells. The anterior swelling is also 
traversed by radial fibres more closely arranged, and radiating, 
fan-like, from the inner membrane to the outer. ‘Their course 
is not straight but curved (fig. 2). Close beneath each 
membrane there is a series of circular muscle-fibres, arranged 
