On Dissotrema papillatum. 3 



differently from the lining cuticle. The greater part of the oral 

 sucker is formed by the muscular fibres, which are arranged in 

 three groups, two of longitudinal fibres and one of radial. Of these 

 the radial fibres are by far the most numerous, traversing the 

 whole thickness of the wall of the sucker between the lining cuticle 

 and the external limiting membrane and leaving comparatively 

 little interspaces between. The individual fibres of this group are 

 very fine, but they are for the most part united into bundles which 

 lightly diverge towards the external limiting membrane. The in- 

 ner longitudinal fibres are found between the bundles of the radial 

 fibres immediately outside the lining cuticle, arranged in radiating 

 groups, each of several layers of fibres, which appear in cross 

 sections of the sucker as so many minute dots. The external 

 longitudinal fibres are also found between the bundles of the radial 

 fibres, but next the external limiting membrane, forming groups 

 which are wider and farther separated from one another, owing to 

 the greater intervals between the bundles of the radial fibres in 

 this part. The bundles of the inner longitudinal fibres are 

 separated from those of the outer by connective tissue. In the 

 interspaces left by the three sets of muscle fibres above mentioned 

 there are small, well stained gland cells, which open by long ducts 

 along the anterior margin of the sucker. 



The oral sucker is accompanied by a pair of dermal glands, 

 each consisting of several cells and opening near the mouth. The 

 cells lie in the parenchyma on either side of the sucker (fig. 1). 



The acetabulum or posterior sucker is an ellipsoidal organ 

 situated close to the hind end of the body and opening on the ven- 

 tral surface by an aperture which is usually more or less I-shaped, the 

 axis of the I being parallel to the long axis of the body. In four 

 total mounts its size was respectively 528x510//, 528x458//, 475 x 

 422// and 528x493//. Its wall is very thick, about 126//, and 

 bounded on the inner side by a cuticle which is the direct continu- 

 ation of that of the ventral side of the body, and on the outer side 

 by a thin limiting membrane separating it from the surrounding 

 parenchyma. The muscular fibres which furm the principal ma*s of 

 the wall of the acetabulum are arranged in three groups, radial, 



