OBSERVATIONS CONCERNING FISH-PARASITES. 105 



doubtless often causes it to fall a prey to some enemy from which it might otherwise 

 easily escape, if the parasite does not at times itself bring about the death of its host. 



One of the most interesting cases of parasitism in which direct injury results 

 to the host, which has come to my attention, is that afforded by the trout of Yellow- 

 stone Lake (Sal mo mykiss). It was noticed by successive parties who visited the lake 

 in connection with government surveys that the trout with which the lake abounded 

 were, to a large extent, infested with a parasitic worm, which is most commonly in 

 the abdominal cavity, in cysts, but which in time escapes from the cyst and tunnels 

 into the flesh of its host. Fish, wheu thus much afflicted, are found to be lacking in 

 vitality, weak, and often positively emaciated. 



It was my good fortune, in the summer of 1890, to visit this interesting region 

 for the purpose of investigating the parasitism of the trout of Yellowstone Lake. The 

 results of this special investigation were published in the Bulletin of the U. S. Fish 

 Commission for 1889, vol. ix, pp. 337-358, under the title: "A contribution to the life- 

 history of Dibothrium cordiceps, a parasite infesting tbe trout of Yellowstone Lake." 



I found the same parasite in the trout of Heart Lake just across the great conti- 

 nental divide from Yellowstone Lake, but did not find any that had tunneled into the 

 flesh of its host, while a considerable proportion of the trout taken in Yellowstone 

 Lake had these worms in the flesh. Some of these worms were as much as 30 centi- 

 meters in length when first removed; others, which had lain in water a few hours 

 after removal before they were measured, were much longer, as much as 54 centimeters. 

 They are rather slender and of nearly uniform size throughout, 2.5 to 3 millimeters 

 being an average breadth of the largest. I found the adult stage in the intestine of 

 the large white pelican (Pelecanus erythrorhynehus), which is abundant on the lake and 

 was found breeding on some small islands near the southern end of the lake. 



In the paper alluded to above, I attempted to account for two things concerning 

 this parasitism among the trout of Yellowstone Lake: First, the abundance of para- 

 sitized trout in the lake; second, the migration of the parasite into the muscular 

 tissue of its host. The argument can not be well summarized in as short space as the 

 requirements of this paper demand. It is sufficient to say that what appear to me to 

 be satisfactory explanations are supplied by the peculiar conditions of distribution of 

 fish in the lakes of this national park. Until three or four years ago, when the U. S. 

 Fish Commission stocked some of the lakes and streams of the Park, the conditions 

 with relation to fish life in the three principal lakes were as follows: Shoshone Lake, 

 no fish of any kind ; Heart Lake, at least three species, Salmo mykiss, Leuciscus atrarius, 

 and Gatostomus ardens ; Yellowstone Lake, one species, Salmo mykiss. Shoshone and 

 Yellowstone lakes are separated from the river systems which drain them by falls too 

 high for fish to scale. Heart Lake has no such barrier. The trout of Yellowstone 

 Lake are confined to the lake and to 18 miles of river above the falls. Whatever 

 source of parasitism exists in the lake, therefore, must continue to affect the fish all 

 their lives. They can not be going and coming from the lake, as the trout of Heart 

 Lake may freely do. If their food should contain eggs of parasites, or if the waters in 

 which they swim should contain eggs or embryos of parasites, they would be contin- 

 ually exposed to infection, with no chance for a vacation trip for recuperation. To 

 quote from my report: 



It follows, therefore, from the peculiar conditions s urronnding the trout of Yellowstone Lake, 

 that if there is a cause of parasitism present in successive years the trout are more liable to become 



