36 ILLIXOIS BIOLOGICAL MONOGRAPHS [316 



sight of the body, then dorsad and anteriad where it opens into the 

 uterus. Mehlis' gland is present altho not well developed, and the cells 

 are scattered along the uterine duct as well as around the ootype, altho 

 they are not so numerous in the former as in the latter location. The 

 vaginae open to the surface on either side at the ventro-lateral margins 

 of the body, at the level of the posterior margin of the ovary (Fig. 16). 

 On either side the inner ends of the vaginae unite just below the ceca 

 with the common ducts from the vitellaria to form the vitello-vaginal 

 canals. These open separately and directly into the ootype. The vitel- 

 laria consist of large compact follicles, underlying the entire dorsal 

 surface of the body from the pharynx to the caudal disc, except the 

 region over the ovary. The vitellaria are reduced and only a few folli- 

 cles are present in the region over the testis and they are entirely absent 

 in a circular area over the ovary. Ventrally the vitellaria do not extend 

 mediad of the ceca. The vitellaria are so extensively developed that they 

 obscure the internal structures and render the body opaque, and this 

 character suggested the name of the species. Common collecting ducts 

 run longitudinally along the body lateral to the intestinal diverticula 

 and these discharge into the vitello-vaginal canals as previously de- 

 scribed. In each of the specimens there is a single large egg in the 

 uterus, and in the one sectioned the uterus extends cephalad of the 

 genital pore and to a point 0.03 mm. from the bifurcation of the intes- 

 tine. The eggs are broadly oval, 0.25 mm. long by 0.2 mm. wide. The 

 shell is yellow, refractive to light, and apparently composed of the same 

 substance that occurs in small droplets in the vitellaria. 



The uterus and cirrus sac open into the genital sinus; the opening 

 of the cirrus is anterior and dorsal to that of the uterus. The common 

 genital pore is situated in the median line, about 0.12 mm. caudad of 

 the bifurcation of the intestine. Embedded in the wall of the cirrus sac 

 and with their points forming the so-called coronet, the genital hooks 

 in appearance suggest the corolla of a flower. There are thirty-three 

 of these hooks in one mounted specimen and thirty-two in the other. In 

 entire length they measure 0.05 mm., the shank or projecting part com- 

 prising about half the total length. 



P. opacum agrees with P. alluaudi and P. orbiculare in shape of 

 caudal disc, but P. alluaudi has but three spines in the genital coronet, 

 and a long post-ovarian uterus which contains many eggs. P. orbiculare 

 has a larger anterior sucker, smaller caudal suckers, a smaller pharynx, 

 fewer vitelline follicles, and only half as many hooks in the genital 

 coronet. P. opacum differs from P. coronatum and P. microcotyle in 

 the shape of the caudal disc and in the reduced condition of the great 

 hooks of the disc. 



