[Diagnostic features of Microsomacanthus microsoma (Creplin, 1829), type species of the genus Microsomacanthus Lopez-Neyra, 1942, as the base for the revision of the genus].

The genus Microsomacanthus was created by Lopez-Neyra (1942). Its type species, Taenia microsoma Creplin, 1829, was described from the common eider Somateria mollissima and remained a collective species until Fuhrmann (1913) revealed its diagnostic features on the original material. Simultaneously he described two new species of "microsoma-Gruppe", Microsomacanthus jaegerskioeldi (Fuhrmann, 1913) and M. diorchis (Fuhrmann, 1913). Fuhrmann stressed that under the name of Hymenolepis microsoma Cohn (1901) gave a description of quite different species (a parasite of the freshwater duck Anas penelope). Much earlier Leuckart (1879) used the same name for the denomination of a certain larval form from freshwater snails, without any experimental verification. Fuhrmann (1932), in conflict with himself, synonymized Taenia microsoma Creplin, 1829 and Hymenolepis microsoma sensu Cohn, 1901 and thus caused new inadequacy. Spassky et Spasskaja (1954) confirmed the validity of the genus Microsomacanthus Lopez-Neyra, 1942 and supplemented its diagnosis with such significant criteria as the number of rostellar hooks (10) and the absence of any accessory structures in the copulatory apparatus. Intermediate hosts of M. microsoma proved to be gammarids (Belopolskaya, 1952). Nevertheless Yamaguti (1959) synonymized a larval form from pond snails (Monocercus lymnaei Villot, 1883) with Microsomacanthus microsoma, meanwhile Hymenolepis microsoma sensu Cohn, 1901 was shown to be a synonym of Microsomacanthus compressa (Linton, 1892). The life cycle of the latter species includes copepods as intermediate hosts and freshwater gastropods as storage (reservoir) ones. Schmidt (1986) and Czaplinski and Vaucher (1994) chose M. compressa as an illustrative example of the genus Microsomacanthus instead of its type species. The diagnostic features of the genus remain vague, so the list of its synonyms attained as much as 17. Fuhrmann's material on M. microsoma, as well as on the two other species of "microsoma-Gruppe", deposited in the Museum of Natural History (Geneva) was reinvestigated. The diagnostic features of the group formulated by Fuhrmann (1913) were confirmed and enlarged. Large assemblage of species corresponding to M. microsoma by morphology (small gonads, early resorption of testes, formation of eggpacket in the uterus) and biology (gammarids as intermediate hosts) is established. This group is taken as the nominotypical subgenus Microsomacanthus (Microsomacanthus). For the antithetic group which life cycle corresponds to that of M. compressa (a species with comparatively large gonads, with testes persisting till the maturation of female gonads and eggs disseminating one by one from the uterus) we erect a new taxon, Microsomacanthus (Leucartcohnacanthus subgen. nov. Microsomacanthus paracompressa (Czaplinski, 1956) is selected as its type species. Representatives of both subgenera of the genus Microsomacanthus may have rostellar hooks of the same length. Literary data on their larval forms were often based on misidentification.