[Microsomacanthus polystictae sp. n. and other cestodes of the family Hymenolepididae (Cyclophyllidea) from Steller's eider Polysticta stelleri from the Chaun lowlands (northwestern Chukotka)].

8 species of cestodes belonging to the family Hymenolepididae were recorded from Polysticta stelleri (Pall.) in the Chaun lowland. A new species of hymenolepidids, Microsomacanthus polystictae sp. n., differing from close (by the length of proboscis hooks) species of the genus in the structure and size of the copulatory organ is described. Brief diagnosis of the new species: cestodes small, specimens ending with hermaphrodite proglottids 2.44 to 2.91 mm long. Proboscis hooks 0.038 to 0.042 mm long, blade 0.011 to 0.012 mm long. Strobila of maximum length with 84 proglottids. In young male proglottids the aporal testis half as large as poral and median ones, in well-developed proglottids testes 0.020 to 0.026 X 0.023 to 0.032 mm. Bursa of cirrus 0.150 to 0.190 X 0.020 to 0.028 mm. Cirrus 0.107 mm long, basis 0.008 to 0.011 mm wide, median part 0.012 to 0.014 mm wide, apical part 0.004 mm wide. Cirrus armed with spines 0.003 to 0.004 mm long. Ovary bilobate, yolk gland compact. Spermatheca small, situated medially, in front of poral ovary lobe. Copulative part of vagina sacciform, 0.086 to 0.118 mm long, 0.023 to 0.037 mm maximum width. Distally and proximally vagina with muscular sphincters. Uterus sacciform, with 35 to 40 embryos. There were no mature "eggs" in the material.