LIFE HISTOEY OF A TROUT PARASITE. 
351 
mature, contained ova, and was evidently identical with those found in the int( stine 
of the pelican. 
This fact confirms what seemed probable from a study of the worms from the 
pelican, viz : that the strobiles are passed in chains from the birds. The excessively 
parasitized condition of some of the trout may thus be accounted for. 
Again, some of the contents of the rectum of a pelican, in whose intestine speci- 
mens of D. cordiceps had been found, was preserved in alcohol for examijiatiou. Upon 
examination of a small portion of this material several ova were found which were 
easily recognized to be ova of D. cordiceps. A small fragment of the strobile of the 
same parasite was also found, which was much frayed evidently by the digestive 
processes of the host. 
It may therefore be taken as demonstrated that both ova and fragments of the 
strobile of T>. cordiceps find their way into the water where they may be swallowed by 
the intermediate host, the trout. 
Of course this argument does not quite amount to a demonstration. If some one 
who has the time, opportunity, and inclination would conduct a series of experiments 
of feeding specimens of trout parasites to ducks, in which they might develop, or, 
better, to pelicans, in which I think they will certainly develop, it would serve to raise 
a part of the history of this worm entirely out of the regions of conjecture. 
There remains also to be ascertained the fate of the eggs of the pelican tapeworm 
after they have been consigned to the water. 
VII.— CONSIDERATIONS RELATING TO THE PARASITISM AMONG THE TROUT 
OF YELLOWSTONE LAKE.- 
In these considerations two problems will be discussed : First, to account for 
the abundance of parasitized trout in Yellowstone Lake; and, second, to account for 
the migration of the parasite into the muscular tissue of its host. 
In order to reach a proper understanding of the matter, a brief review of some of 
the physical features of Yellowstone Lake and the surrounding region is necessa/y. 
Yellowstone Lake is a large body of water of very irregular outline, containing about 
150 square miles of surface and having a coast line of approximately 100 miles. It 
lies near the great continental divide and empties through the Yellowstone Eiver into 
the Missouri-Mississippi River system. About J.8 miles below the lake there are two 
falls in the river. The upper fall is 109 feet in height, the lower, which is one-half 
mile farther down stream, is 308 feet in height. On the western side of the continen- 
tal divide there are three lakes, much smaller than the Yellowstone, but still quite 
considerable bodies of water. These are called Heart, Lewis, and Shoshone, respect- 
ively, and each is not more than 8 miles in an air line from the nearest point on 
Yellowstone Lake. Lewis and Slioshoue Lakes empty through Lewis River into Snake 
River, a tributary of the Columbia. Between them and Snake River there are falls of 
some 60 feet in height. Heart Lake empties into Snake River through Heart River, 
but there are no falls on this latter stream. 
The natural distribution of fish in these lakes presents some peculiarities which 
should be mentioned here. In Lewis and Shoshone Lakes there are no fish. This is 
not due to an absence of food nor to the presence of conditions unfavorable to life, 
since these lakes were found to be swarming with amphipods, entomostracans, and 
