106 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



infested than they would be iu waters where they had a more varied range. Trout would become 

 infested earlier and iu greater relative numbers, and the life of the parasites themselves — that is, their 

 residence as encysted worms — must be of longer duration than would be the rule where the natural 

 conditions are less exceptional. There are probably not less than one thousand pelicans on 



the lake the greater part of the time throughout the summer, of which at any time not less than 50 

 per cent are infested with the adult form of the parasite, and, since they spend the greater part of 

 their time on or over the water, disseminate millions of tapeworm eggs each in the waters of the 

 lake. It is known that eggs of other dibothria hatch out in the water, where they swim about for some 

 time, looking much like ciliated infusoria. Dounadieu found in his experiments on the adult dibothria 

 of ducks that the eggs hatched out readily in warm water and very slowly in cold. If warm water, 

 at least water that is warmer than the prevailing temperature of the lake, is needed for the proper 

 development of these ova, the conditions 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 three miles the shore is 

 skirted by a hot spring and geyser formation with numerous streams of hot water emptying into the 

 lake, and large springs of hot water opening in the floor of the lake near shore. 



Trout abound in the vicinity of these warm springs, presumably on account of the abundance of 

 food there. They do not love the warm water, but usually avoid it. Several persons with whom I 

 talked on the subject while in the park assert that diseased fish — that is to say, those which are thin 

 and affected with flesh- worms — are more commonly fouud near the warm water; that they take the 

 bait readily, but are logy. I frequently saw pelicans swimming near the shore in the vicinity of the 

 warm springs on the west arm of the lake. It would appear that the badly infested or diseased fish, 

 being less active and gamy than the healthy fish, would be more easily takeu by their natural enemies, 

 who would learn to look for them in places where they most abound. But any circumstances which 

 cause the pelican and the trout to occupy the same neighborhood will multiply the chances of the 

 parasites developing in both the intermediate and final host. The causes that make for the abundance 

 of the trout parasite conspire to increase the number of adults. The two hosts react on each other 

 and the parasite profits by the reaction. About the only enemies the trout had before tourists, 

 ambitious to catch big strings of trout and photograph them with a kodak, began to frequent this 

 region, were the fish-eating birds, and chief among these in numbers and voracity was the pelican. It 

 is no wonder, therefore, that the trout should have become seriously parasitized. It may be inferred 

 from the foregoing statements that the reason why the parasite of the trout of Yellowstone Lake 

 migrates into the muscular tissue of its host must be found in the fact that the life of the parasite 

 within the fish is much more prolonged than is the case where the conditions of life are less excep- 

 tional. 



The case just cited is probably the most signal one of direct injury to the host from 

 the presence of parasites that I have seen. I shall enumerate more briefly a few 

 additional cases out of a great number that I have encountered in my special investi- 

 gations on the entozoa of fishes for the United States Fish Commission. 



The codfish very commonly harbors adibothrium(T). rugosum), which fixes itself 

 firmly in a branch of the pyloric cceca, the chain of proglottides extending into the 

 intestine. In all cases that I have seen the head and anterior portion of the body of 

 the worm have been firmly impacted in a yellowish, waxy secretion, which is, in part 

 at least, derived from degenerated tissue of the coecal appendage. These parasites 

 must interfere seriously with the digestive processes of their host, and therefore by 

 jnst so much impair its vitality. I have found the striped bass (Roccvs lineatus) 

 about Woods FIoll, Mass., almost without exception, infested with a thorn-head worm 

 (Echinorhynehm proteus). These parasites are always attached, often to the number 

 of several hundred, firmly to the walls of the large intestine of their host. In most 

 cases the thorny proboscis and inflated portion of the neck have penetrated the intestinal 

 walls and protrude into the body cavity, where they are covered by a waxy secretion 

 from the tissues of their host. This boring process gives rise to considerable inflamma- 

 tion, and the rectum of the fish is usually much inflamed. Evidently 1 hese parasites, in 

 addition to the disadvantage to which they subject their host by their passive presence, 



