116 TWENTY-FIRST REPORT. 



The entire basin of Hudson's Bay and its tributary streams, so far as 

 known, is also occupied by the Mississippi fauna. This area was peopled by 

 an early post-glacial migration from the Mississippi valley at the time when 

 glacial Lake Agassiz emptied south through that river. 



Detroit, Michigan. 



NOTES OF THE PRESENCE OF LARVAL TREMATODES IN THE EYES 

 OF CERTAIN FISHES OF DOUGLAS LAKE, MICHIGAN. 



BY E. PRISCILLA BUTLEB. 



(Abstract.) 



In the summer of 1917, young Herring Gulls which were fed upon dead 

 fish picked up on the beach of Douglas Lake became very highly infested 

 with trematode parasites. Among these were certain Holostomidae, belonging 

 to the genus Strigea and to an undetermined genus. In the following sum- 

 mer search was made for the larval forms of these trematodes in the fishes 

 of the lake. Larval Holostomidae of the generic groups Diplostomulum and 

 Tetracotyle were found in large numbers in the eyes of certain of the fishes. 



Seventy-nine specimens, belonging to seven species, of fish were examined 

 for parasites. Fifty per cent of the Trout Perch, sixty per cent of the Rock 

 Bass, and seventy-two per cent of the Yellow Perch examined were found to 

 be so infested. The data secured from the other species, namely the Com- 

 mon Bullhead, the Common Sucker, the Common Shiner, and the Pumpkin- 

 seed, are not sufficient to warrant any conclusion as to the degree of infesta- 

 tion of these species, because of the small number of specimens examined. 



All the parasites found in the eyes were in the free living state. They 

 occurred within the pupil, in the anterior and posterior chambers, on the iris, 

 in the pigmented tissue of the choroid lining, and, most commonly, in the 

 vitretous humor. Their presence in such large numbers indicates that the 

 eyes of these fish form a natural habitat for Diplostomulum and Tetracotyle. 



European species of Diplostomulum and Tetracotyle which infest fish are 

 known to develop into adult forms, Hemistomum and Strigea respectively, 

 which are parasitic on water-birds. The relation of the larval trematodes 

 found at Douglas Lake in 1918 to the adult forms taken from the Herring 

 Gulls the previous summer is not definitely proved, but is strongly sus- 

 pected. 



The early development of Diplostomulum and Tetracotyle, before the 

 stages found in the fish, is not known. 



Further investigation of this problem is planned. 



University of Michigan. 



