524 
HENRY LESLIE OSBORN 
usual longitudinal circular and oblique fibres can be seen. In 
tangential sections the fibres seen are scanty and very small. 
On the other hand the suckers have more than the usual size. 
The dimensions of the oral sucker have already been noted. Its 
histological structure has the usual characteristics, the mass of 
the organ being a very thick wall made up principally of radial 
muscle fibres among which at wide intervals there are a few 
nuclei. 
The ventral suckers show well in figs. 1,3, and 5. They are of 
the same size and lie in the median plane. The anterior one is 
more ventral and nearer to the surface of the animal, it is also 
more nearly opposite the opening of the sheath, as shown in fig. 
3. In fig. 1 the lip of the sheath can be seen cutting across the 
outlines of both suckers. Posteriorly the body wall folds in 
a considerable distance to reach the posterior sucker as in 
fig. 5. 
In addition to the possession of two ventral suckers of equal 
value 'this apparatus is still further aberrant, inasmuch as it is a 
semi-internal organ, made so by the development of a sheath or 
pocket into which the suckers are withdrawn, so as to be w^holly 
immersed within the contour of the surface, where they can be 
partly enclosed by a sphincter muscle located in the margin of the 
sheath. In living worms the lips of this sheath are at times con- 
siderably dilated and the suckers exserted, at other times the lips 
are contracted. The mechanism of this movement seen in section 
consists of a band of muscular fibres shown at d in fig. 3 and ms 
in fig. 5. This sheath is a totally unique anatomical feature as 
well as the two ventral suckers. 
So far as I am aware, such a peculiarly constructed ventral 
sucker apparatus is quite unique in distomes. A second case of 
a fluke with two ventral suckers has been reported, that of Podo- 
cotyle (Luehe, '00) but the structure of that form is entirely 
different from this, one of the suckers being located on a long 
stalk and there being no sheath. In no other instance is there 
such a sheath with sphincter muscle lodging a second fully devel- 
oped sucker. There are no data at hand by which to decide 
which of the ventral suckers should be homologized with the van- 
