[The state of knowledge of the class of trematodes and the chief problems in studying them].

The paper deals with the current state of knowledge of Trematoda, trends and aims of further investigations. Up to now the classification system of trematodes has been unsatisfactory though for several large groups it is elaborated with due regard to the biology and ecology of these worms. The main deficiency of the present-day taxonomy of trematodes consists in the existence of two separate classifications, for adult worms and for their larvae (cercariae). Among the existing systems the classification suggested by Odening (1960-1963) is of the greatest interest. We do not agree with his division of the class into three subclasses (including Aspidogastrea) and Diginea into two subclasses, "Sporocystoinei" and "Redioinei", but follow the composition of orders established by him. Great difficulties arise in establishing the characters, taxonomic criteria of trematodes. However, a number of characters can be suggested for differentiation of species and genera at various developmental stages. Deficiency in the taxonomy of these worms is due to a wrong evaluation of their characters and to the lack of common collection and treatment techniques. On describing species one should proceed, if possible, from the whole complex of morphological and biological peculiarities of different ontogenetic stages of the forms under study. A study of the ultrafine structure, hysto- and biochemistry of trematodes will enable us to associate better the morphology and the function of organs and to go deeper into the functional morphology and functional biochemistry. The main task of further investigations is closely connected with the elaboration of a natural system of Trematoda, a study of diverse and deep interrelations in the "parasite-host" system and with the development of scientific methods of control of pathogenic species on the biological basis.