REPORT OF COMMISSIONER OF FISH AND FISHERIES. 139 
lected, but they may abound earlier in the season, in temporary pools. 
Entomostraca were abundant in every situation where 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 proportion of animal life found in the streams consisted of 
insect larve, chiefly neuropterous. Caseworms, ephemerid larvae, and 
the larvee of stone flies and Stalida, which are everywhere excessively 
abundant, are the main dependence of the trout in waters containing no 
other fishes. The larvie of Chironomus are very common, and those of 
Simulium are exceedingly numerous in suitable localities. The smaller 
mollusks, especially species of Physa, were also plentiful occasionally. 
In the deepest waters examined the bottom fauna consisted mainly 
of a few slender annelds, an abundance of red Chironomus larvee, Some 
small mollusks, mostly Pisidiwm, and occasional specimens of Gamma- 
rus. By far the most important elements of fish food, however, were 
the entomostraca for the young and the neuropterous larve 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 
park, and has ever since attracted much attention from tourists and 
Scientific men, The parasite is a species of tapeworm, named Diboth- 
rium cordiceps 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 complete its life history. The latter is, 
in part at least, the white pelican, which spends 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 per cent or more are infested 
with the adult Dibothriuwm, 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 in such places as the shore system of geysers and hot springs on the west 
arm of the lake, where for a distance of nearly 3 miles the shore is skirted by a hot 
