350 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
where they give rise to ciliated emliryos, which bear a close resemblance to ciliate 
infusoria. These pass into fishes, particularly the CyprinoidSj where they become 
established in the peritoneal cavity. The round of life is completed in the intestines 
of aquatic birds, whore the eggs are rapidly formed. 
VI.— REASONS FOR REGARDING THE PELICAN AS THE FINAL HOST OF ’ 
D. CORDICEPS. 
While th'ere seems to be no reason to doubt that the tapeworm found in the 
pelican is the adult of the trout parasite, it may be well to sum up the evidence which 
has led to that conclusion. 
In the first place, it is to be noted that the parasite of the trout is a true larva and 
shows no signs of assuming the adult condition in the trout. Even the largest speci- 
mens, which have left their blastocysts and migrated into the tissues of their host, 
show but the faintest beginnings of the reproductive organs. Moreover, no cestod 
is known to attain the adult condition elsewhere than in the alimentary canal of its 
proper host. Again, out of the large number of trout examined there was not a single 
case ot a mature worm of this genus in the alimentary canal. A few of the large trout 
of Yellowstone Lake were convicted of cannibalism, since their stomachs contained 
remains of trout, the only species of fish in the lake. There is no doubt that a large 
number of trout of the lake are eaten by larger fish of the same species. If the flesh 
worm of the trout ever matures in the intestine of the trout, or, in other words, if 
the trout is both intermediate and final host of this parasite, I should have found 
some evidence of it. Failing to find the adult in the trout, search had to be made 
among the animals which feed on the trout. A very brief consideration of the fauna 
of the Yellowstone region was sufiicient to make it clear that the adult form of this 
worm must live in the intestine of some of the fish-eating birds that inhabit the lake. 
Moreover, since the cause of infection must extend through several months of the year, 
as shown by the variety of sizes of parasites occurring in a single fish, the final host 
is seen to be more probably a bird.that stays through the summer than one which is 
only a visitor. F^elicans abound on the lake, one or more of them being usually in 
sight, on any part of the lake, at any time of day during the summer months. They 
have numerous roosting-places and at least one breeding place on the lake. They are 
known to be notorious 'fish-eaters. It is clear, therefore, that collateral evidence alone 
points strongly to the pelican as a, if not the, final host of the flesh worm of the trout. 
Evidence of a more direct nature, however, was obtained bythe capture and exam- 
ination of four pelicans. Their stomachs contained partly digested remains of large 
trout, and practically nothing else, thus demonstrating their ability to capture the large 
trout of the lake, and showing that they live exclusively on a fish diet ; in the intes- 
tines of two of them were tapeworms, which there is no reason to doubt are the adult 
stage of the trout parasite. When one of these worms fromThe pelican is compared 
with a parasite from the flesh of the trout, the head with its characteristic botliria, 
or pits, is found to be practically unchanged. The strobile, or jointed body of the 
worm, is, as might be expected, much longer and lai’ger in every way. There do not 
appear, however, any characters in one that are contradicted in the other. 
On August 9, on the shore of the southeast arm of Yellowstone Luke, I picked 
up at the edge of the water three fragments of a Dibothrium strobile. The largest of 
these fragments was 70 millimeters in length and 6 millimeters in breadth. It was 
