Characteristic of life cycles of trematodes (Trematoda) of land vertebrates from the Middle Volga region

199 trematodes species are registered at amphibians, reptiles, birds and mammal of the Middle Volga region for more than 100-years period of helminthological researches. It is established that development of165 species of trematodes with the investigated and most probable life cycles in the region proceeds on24 types and 11 subtypes from 3 groups of life cycles. Their greatest part circulates on complex – trixenic (125 species) and tetraxenic (6) - cycles; a smaller one – on dixenic cycles (34 species). Cycles of development are not established or completely investigated for 34 trematodes species of land vertebrates of the Middle Volga region.Amphibians participate as final hosts in circulation of 25 trematodes species; as additional and/or reservoir ones - of 8 species of helminths of reptiles, 15 species - of birds and 4 – of mammals. Reptiles as final hosts participate in circulation of 14 trematodes species; as additional and/or reservoir ones – of 5 species of helminths of birds and 2 – of mammals. Birds as final hosts participate in circulation of 105 trematodes species. Mammals are final hosts for 61 trematodes species and micromammals serve as reservoir ones for Alaria alata.Trematodes of birds are characterized by the greatest variety of types of life cycles (9 types and 7 subtypes); in a lesser degree – amphibians (6 types and 2 subtypes), mammals (7 types and 5 subtypes) and reptiles (3 types and 4 subtypes).