SEA-FISHERIES LABORATORY. 137 



parasite is often a useful indication of the places of origin 

 of fishes with periodic migratory habits. 



The following description of the trematode may be 

 useful, since the species is not at all fully described in the 

 literature. It is based on a reconstruction from a com- 

 plete series of sections made from a worm of 9'7 mm. in 

 total length, the largest found. The trematode was killed 

 by immersion in fresh water, and was subsequently pre- 

 served in formalin, a procedure which enables one to 

 preserve the worm with the minimum amount of dis- 

 tortion. But such a method is not favourable for the 

 study of histological details, and I deal here only with 

 the coarser anatomy of the parasite. 



The measurements of the specimen are: — 



Total length : 97 mm. 



Greatest breadth : 2 mm. 



Transverse diameter of oral sucker : 0*76 mm. 

 „ ,, ventral sucker : 094 mm. 



Diameters of ova: O079— 0'095 by 0'048— 

 0'064 mm. 



Transverse diameter of oral sucker is contained 

 12 times in total body length. 



Transverse diameter of ventral sucker is con- 

 tained 9| times in total body length. 



Odhner's measurements vary slightly from those 

 given above. Thus in his fig. 11, Taf. 33 {op. cit.) the 

 body length is 10| times the transverse diameter of the 

 oral sucker, and 6|- times the transverse diameter of the 

 ventral sucker. These ratios are not of precise diagnostic 

 value ; thus in a smaller specimen examined I found that 

 the diameter of the oral sucker was contained about 10 

 times in the body length, and that of the ventral sucker 

 about 7 times. Neither are the diameters of the ova very 

 constant. Odhner gives these as 0'07— 0'08 by 0'037. 



