444 THE AMERICAN NATURALIST [Vol. LVI 



The primary host, however, is not definitely known. 

 Professor Linton found that in the case of the infection 

 of the trout of Yellowstone Park the white pelican acted 

 as the primary host, and, in the light of this finding, it 

 is quite probable that some similar fish-eating bird is the 

 primary host of the larval tapeworm under discussion. 



While at Cooper lake a canvass was made of the com- 

 mon fish-eating birds which visit , the lake, and it was 

 found that the blue heron is the most frequent visitor. 

 Since no pelicans are know to come to the lake, I rather 

 strongly suspect that the blue heron acts as the primary 

 host for the larval tapeworms of the trout. If this should 

 prove to be the case then the life history, in all prob- 

 ability, would be as follows : The adult Diphijllobothrium 

 tapeworm develops in the intestinal tract of the blue 

 heron, and when the segments become mature they are 

 periodically passed out with the faeces. These mature 

 segments contain large numbers of developing embryos 

 and if they are deposited in a stream or lake the embryos 

 are swallowed by the fish, in which they develop into the 

 larval tapeworms already described. When a blue heron 

 captures one of these infected fish, the larvjr attach them- 

 selves to the bird's intestinal wall and shortly develop 

 into adults capable of carrying on the life cycle. 



My visit to Cooper lake convinced me that it was pure 

 folly to entirely close down a lake for more than a year 

 or two. In the first place, closing down a lake makes 

 for a rapid increase of fish so that the available food 

 supply soon becomes inadequate for maintaining all of 

 them, with the result that a fierce struggle for existence 

 ensues, in which many of the weaker, but nevertheless 

 desirable, fish are killed off. Even those which survive 

 in the struggle appear to be starved. Secondly, when a 

 lake is closed its shores afford an ideal, undisturbed nest- 

 ing place for such fish-destroying birds as the blue heron, 

 kingfisher and the like. These birds not only destroy 

 large numbers of fish, but they may be the means of dis- 

 seminating parasitic infections. And lastly, in the light 



