Variability and systematic status of some species of the trematodes of genus Pachytrema (Pachytrematidae) parasitizing in fish-eating birds

The morphometrical variability of Pachytrema calculus Looss, 1907 from Laru s ridibundus, Podiceps nigricollis, Tringa nebularia and Chlidonias niger, Pachytrema paniceum Bringmann, 1942 from Larus ridibundus, Pachytrema compo situm Tscherbowitsch, 1946 from Sterna hirundo in the Czech Republic, Pachy trema sanguineum (Linton, 1928) from Larus atricilla in Cuba, and Pachytrem a magnum Travassos, 1921 from Thallasseus magnus in Brazil was studied. Pachytrema magnum, Pachytrema paniceum and Pachytrema sanguineum are consid ered to be junior synonyms of Pachytrema calculus. Pachytrema compositum fr om Sterna hirundo differs from the foregoing species in a distinctly smalle r size of eggs (0.080 x 0.040 for Pachytrema compositum against 0.114 x 0.0 54 for above mentioned spesies).