"'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 found 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 taken 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 exceptional. "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 investigations on the entozoa of fishes for the U. S. Fish Commission." Many worms of this type abound in codfishes, bluefishes, striped bass, and other marine fishes, rendering them lean and unfit for food. =The Heart Lake Tape-worm.=--Another very interesting case of parasitism is that of the large tape-worm (_Ligula catostomi_) infecting the suckers, _Catostomus ardens_, in the warm waters of Witch Creek, near Heart Lake, in the Yellowstone Park. Of this Dr. Linton gives the following account: [Illustration: FIG. 230.--Sucker, _Catostomus ardens_ (Jordan & Gilbert), from Heart Lake, Yellowstone Park, infested by a flatworm, _Ligula catostomi_ Linton, itself probably a larva of _Dibothrium_. (After Linton.)]