FISH ENTOZOA FROM YELLOWSTONE PARK. 553 



TREMATODA. 



Encysted Distoma from Leucisccs atkakius. 

 [Plate 65, Figs. 22-2.5.] 



On July 30 I examined several small chubs {Leueiscus atrarius) from 

 a warm branch of Witch Creek, and some larg^er lish of the same species 

 from Heart Lake. In these fish the mesentery and serous covering of 

 the viscera generally were thickly specked Avith 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 larvie were observed 

 in one cyst, and in several others two. The larvai lay in a transpar- 

 ent fluid which was inclosed in an oval sac 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 0.2G 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- 

 reaUtm, but there is no indication of the head papillse 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 i)ore of the water- vascular system. 

 From a short median \essel connecting with the terminal pore a. num- 

 ber of marginal vessels branch like the arms of a candelabra. The 

 ventral sucker is well developed, the oral im])erfectly so. 



DiSTOMlIM LAUHKATUM ZedOT. 



Rudolphi Entozoa Hist., ii, 413; id., Syiiops., 113, 413; Diesing, Syst. Helminth., 

 380; Dn.jardin, Hist. Helminth., 435; Oisson, Bitrag till Skandinavicns 

 Helminthfauna, 1876, 24, Taf. iv. Figs. 52, 53, 54. 



Body unarmed, depressed, oblong, with short neck. Head with four 

 low, blunt, -dorsal papilhe and two ventro-marginal lobes on oral sucker. 

 Ventral sucker larger than month, sessile but prominent, with trans- 

 verse cleft-like aperture. Pharynx globose near the mouth, with short 



