by james nelson gowanloch 13 1 



Summary 



1. Observations on the occurrence and activity of Gyro- 

 dactylus, a monogenetic trematode occurring as an ectoparasite 

 on Fundulus heteroclitus in the Bay of Fundy and in the neigh- 

 borhood of Halifax, N. S., are here reported. 



2. Gasterosteus also serves as host but Pseudopleuro- 

 nectes americanus proved not to be susceptible. 



3. The trematodes remained apparently unaffected by a 

 series of lowered salinities ranging down to ten parts in a thous- 

 and but exposure to the action of either fresh water or distilled 

 water causes the rapid migration of the parasites from the hosts 

 and their death. 



4. The toxic action of tap water was found to be lethal 

 in about five hours, that of distilled water lethal in about twenty 

 minutes. 



5. The haemoglobin of the parasitized fish was found 

 to be reduced in some instances to less than one twelfth of the 

 normal value, the average for the series examined giving a 

 reading of only forty per cent of the value shown by normal fish. 



6. The direct cause of death was found to be the loss of 

 blood from the innumerable minute perforations which the 

 parasites make during their feeding activities. 



7. Treatment of affected fish with fresh-water is an 

 evidently available means of destroying the parasites. 



