246 American Fisheries Society 



words eater and eaten. Such, for example, is the relation 

 between the hosts of a small tapeworm (Otobothrium crena- 

 colle), which is adult in certain species of shark, and is 

 found encysted in a large number of our bony fishes, notably 

 in the common butterfish {Poronotus triacanthus) , where it 

 is encysted, sometimes in enormous numbers, in the mus- 

 cles. (During this past summer I removed 7,932 cysts from 

 the muscles of a single butterfish. ) Another, which brings 

 together a bird as the eater and a fish as the eaten, is that of 

 a tapeworm (Dibothriitui cordiceps), larval in the Rocky 

 Mountain trout and adult in the white pelican. 



Indeed, birds and fishes, in the economy of nature, have 

 long been associated as eater and eaten, and it should not be 

 a matter of surprise, therefore, that fishes should be inter- 

 mediate hosts of many parasites whose development into 

 adult, egg-producing worms depends upon their entering the 

 alimentary canal of some bird. 



Not only cestodes, or tapeworms, but trematodes, or 

 flukes, as well, are known which have fishes for their inter- 

 mediate hosts and birds for their final hosts. The life 

 history of trematodes, however, is more complicated than 

 that of the cestodes, in that an invertebrate, usually a mol- 

 lusc, serves as one of the intermediate hosts. One of the 

 few examples of trematode life-histories which have been 

 worked out is that one, now become a classic, furnished by 

 the liver fluke of the sheep ( Fasciola hepatica ) . It is scarcely 

 possible to give a satisfactory synopsis of this life-history 

 without figures and the use of technical terms. Briefly it is 

 as follows : 



The adult flukes live in the biliary ducts of the sheep's 

 Hver. The eggs pass readilv into the intestine and thence, 

 along with the faeces of their host, to the exterior. Falling in 

 moist places there hatch out from the eggs ciliated embryos 

 of microscopic size, 1/200 of an inch, more or less, in length. 

 They live for a time in the water swimming about actively. 

 Coming in contact with a species of fresh-water snail they 

 enter its pulmonary chamber, where they become fixed, lose 



