REPORT OF COMMISSIONER OF FISH AND FISHERIES. 139 



lected, but they may iilxmiid earlier in tlie season, in temporary ])Ools. 

 Entomostraca were abundant in every .situation wlierc they might 

 naturally be expected to occur, copepods largely predominating, except 

 in the smaller ponds. These indispensable elements to the preservation 

 of young fishes were as plentiful in the waters of the park as they are 

 in Lake Michigan and in the deeper lakes of the Wisconsin series. 



The greater i)roY)ortion of animal life found in the streams consisted of 

 insect larvoe, chiefly neuropterous. Caseworms, ephemerid larvse, and 

 tlie larvte of stone flies and SialUla', which are everywhere excessively 

 abundant, are the main dei^eudence of the trout in waters containing uo 

 other fishes. The larvfe of Ghironomns are very common, and those of 

 Simnlium are exceedingly numerous in suitable localities. The smaller 

 mollusks, especially species of Physa, were also j)lentiful occasionally. 

 In the deepest" waters examined the bottom fauna consisted mainly 

 of a few slender annelids, an abundance of red CMronomtis larvae, some 

 small mollusks, mostly Pisidium, and occasional specimens of Gamnia- 

 rtis. By far the most important elements of fish food, however, were 

 the entomostraca for the young and the neuropterous larva3 for the 

 adult fishes. 



Prof. Linton was entirely successful in his study of the wormy trout, 

 the history of which he has been able to complete. This instance of 

 excessive parasitism was noticed during the early explorations of the 

 jiark, and has ever since attracted much attention from tourists and 

 scientific men. The parasite is a species of tapeworm, named Dihoth- 

 riuni cord'tceps by Prof. Joseph Leidy, and occurs among the viscera, 

 beneath the peritoneal lining of the abdominal cavity, or burrowing 

 in the muscular tissue of the body wall. The only fish which it is 

 known to infest is the native trout of the Yellowstone Park, and it has 

 been found almost exclusively in Yellowstone Lake, in Yellowstone 

 River above the falls, and in Heart Lake. It does not, however, finish 

 its development in the trout, which contain only the larval stages, 

 but requires a second host to comj)lete its life history. The latter is, 

 in part at least, the white pelican, which sjiends the summer in this 

 region, and breeds on an island in the southeast arm of Yellowstone 

 Lake. Ail of the birds examined had been feeding on the trout, the 

 only fish occurring in the lake. 



The cause of the unusual multiplication of parasites in this locality 

 may be traced to the peculiar combination of circumstances there pre- 

 vailing. Probably not less than 1,000 pelicans resort to the lake dur- 

 ing the summer, and of this number 50 i^er cent or more are infested 

 with the adult JHbothnum, the eggs of which become disseminated 

 through the water, where, after a short development, they are swal- 

 lowed by the trout. The eggs hatch more readily in warm than cold 

 water, and the former conditions, according to Prof. Linton, are — 



Supplied ill siu'li places as the shore system of geysers ami liot spriugs on the Avest 

 arm of the lake, where for a distauco of nearly 3 miles the shore is skirted hy a hot 



