No. 406.] AMERICAN FROGS. 813 
There are two testes, somewhat elongated or of slightly 
irregular outlines, situated one upon either side of the body, 
about in line transversely with the ventral sucker. The 
penis sac is a prominent organ at the left of the ovary and 
having a length of about one-fourth the length of the worm. 
It has thick muscular walls within which the retracted penis 
lies coiled. 
The vagina, or terminal portion of the oviduct, opens imme- 
diately beside the opening from the penis sac and in front of 
it. The two openings appear in many specimens as separate 
apertures upon the surface of the body; in others they open 
into a common groove or depression of the body wall or genital 
atrium. 
The eggs are elliptical or very slightly ovoidal in outline, 
measuring about 23 x 13 4. The line of separation of the lid 
is usually not evident. 
The excretory vesicle forks just in front of the excretory pore, 
forming two tubules which diverge as they extend forward near 
the testes, beyond which I have not traced them. 
It will be seen from the foregoing description that D. arca- 
num is closely related to several species which it resembles 
in its small size, compact form, spiny covering, rudimentary 
intestine, and laterally placed sexual aperture, as well as in 
being parasitic in the intestine of the frog. D. medians is the 
form which it resembles most closely, the two species being 
essentially alike in many respects. D. medians has, however, 
a regular rounded ovary, situated on the right side, while, as 
already stated, the ovary in D. arcanum is lobulated and nearly 
median in position. The latter species appears also to be a 
little smaller and less elongated. The vitellaria, also, are dis- 
tributed over a zone whose breadth is about one-half the length 
of the worm, while 2. medians has the vitellaria restricted to 
the anterior quarter of the animal. Measurements of the eggs 
of the two species show that they also differ in size. There is 
further to be taken into account the fact that D. medians lives 
free in the intestine in (European) frogs, while D. arcanum is 
found completely enclosed within cysts at the pylorus of 
