222 American Fisheries Society 



The cestocles are in general far larger than trqmatodes 

 and play accordingly a more important role. They live in 

 the intestine, in the pyloric appendages, or ca?ca, or even in 

 the body cavity. One finds them often in such size and num- 

 bers that the cavity of the canal appears to be stuffed full 

 and the intestinal wall is markedly distended by their bodies. 

 Tn such cases the effect of their presence has been noted by 

 many observers in the greatly emaciated body of the fish. 

 The most frequent types of such worms are the Ichthyotas- 

 niadse and the Bothriocephalida?. An extensive monograph 

 on the former family has been completed by one of my stu- 

 dents and is to be published within the year. His preliminary 

 record of the forms discovered has already appeared (La 

 Rue, 1911). 



In the body cavity other species of tapeworms are to lie 

 found in certain fresh-water fish. Linton has worked out 

 the life history of one such species, Dibothriam cordiceps, 

 which occurs in the trout of Yellowstone Lake, and is so 

 abundant that it prevents the use of these fish for food, ex- 

 cept by the pelicans that fish there and in return for their 

 labor secure not only a meal of trout but also a good supply 

 of tapeworms. Similar parasites, known as Ligitla, are 

 found in fish of the carp tribe in Europe and lead to the 

 death of many of their hosts. Another form, Schisto- 

 ccphalus dimorphus, occurs frequently in the body cavity of 

 the European stickleback and at times destroys great num- 

 bers of these fish. I have myself observed at Loring, Alaska, 

 an epidemic among the sticklebacks which was caused by a 

 tapeworm in the body cavity that belonged to the same or a 

 very closely related species. These forms are, however, 

 generally distributed in this country. Leidy (1855 ) was the 

 first to report them from the Atlantic coastal region. 



Round- or threadworms are the most common parasites 

 of fishes, and in number of species they exceed other internal 

 parasitic worms. One finds them both free in the alimentary 

 canal and encysted in the various organs of the body. They 

 are mostly small and very uniform in appearance. Tn spite of 



