FISH ENTOZOA FROM YELLOWSTONE PARK. 553 
TREMATODA. 
ENCYSTED DISTOMA FROM LEUCISCUS ATRARIUS. 
[Plate 65, Figs. 22-25. ] 
On July 30 I examined several small chubs (Leuciscus atrarius) from 
a warm branch of Witch Creek, and some larger fish of the same species 
from Heart Lake. In these fish the mesentery and serous covering of 
the viscera generally were thickly specked with minute white granules, 
which upon examination proved to be cysts containing larval trema- 
tods. In most cases each cyst contained but one larva, but in one 
instance, among cysts from a large chub, three larve were observed 
in one cyst, and in several others two. The larvee lay in a transpar- 
ent fluid which was inclosed in an oval sae with pellucid walls. The 
sac collapses when the larva is liberated. It is then seen to consist 
of interlacing fibers, the interstices of which are filled with granular 
parenchyma. The parasites are too immature to make their identifica- 
tion certain. I record, however, the following characters: 
The average length of the cysts appears to be about 0.5 mm. An 
alcoholic cyst measured 0.7 mm. in length and 0.5 mm. in breadth, and 
contained a larval distomum 0.4 mm.long and 0.25 mm. broad. Another 
larva, liberated from a somewhat smaller cyst, measured 0.32 and 0.16 
mm., and another 0.35 and 0.26 mm. in length and breadth, respectively. 
Another was 0.4 mm. long and 0.15 mm. thick. 
These distoma are elliptical, oblong, or oval in outline when viewed 
dorsally or ventrally, but in marginal view are convex dorsally and 
concave ventrally. The two suckers have made their appearance, and 
also the muscular pharynx. These organs have about the same rela- 
tive positions and proportions as the similar organs in Distomum lau- 
reatum, but there is no indication of the head papille of that species. 
The body is somewhat longitudinally striated behind the ventral 
sucker, an appearance which seems to be due to the branches of the 
water-vascular system. There is a posterior emargination which 
marks the position of the terminal pore of the water-vascular system. 
From a short median vessel connecting with the terminal pore a nun- 
ber of marginal vessels branch like the arms of a candelabra. The 
ventral sucker is well developed, the oral imperfectly so. 
DISTOMUM LAUREATUM Zeder. 
Rudolphi Entozoa Hist., 11, 413; id., Synops., 118, 413; Diesing, Syst. Helminth., 
380; Dujardin, Hist. Helminth., 435; Oisson, Bitrag till Skandinaviens 
Helminthfauna, 1876, 24, Taf. 1v, Figs. 52, 53, 54. 
Body unarmed, depressed, oblong, with short neck. Head with four 
low, blunt, dorsal papilla and two ventro-marginal lobes on oral sucker. 
Ventral sucker larger than mouth, sessile but prominent, with trans- 
verse cleft-like aperture. Pharynx globose near the mouth, with short 
