A new monotypic genus, Hemiacanthomysis, is established for Acanthomysis dimorpha Ii, 1936. The new genus is distinguished from Acanthomysis and its related genera by the endopod of the uropod, which is armed with numerous spines along the inner margin, and the telson, which is armed with subequal spines all along its lateral margins. A complete redescription is provided for the male of H. dimorpha, comb. nov.
The pyurid ascidian genus Herdmania Lahille, 1888 has been known in Japanese waters by a single species, H. momus (Savigny, 1816), with two later proposed taxa treated as its junior synonyms. My detailed examination of many museum specimens, including most of those recorded from Japan in earlier publications and name-bearing types of the species to be compared, and also of newly collected material, reveals that these waters are inhabited by the following seven species : H. momus (Savigny), s. str. ; H. pallida (Heller, 1878), for which a lectotype is designated ; H. japonica (Hartmeyer, 1909), based on examination of syntypes, with Rhabdocynthia siphonalis Oka, 1933 as a probable junior synonym ; H. mauritiana (Drasche, 1884) (=H. insolita Monniot and Monniot, 2001), based on examination of the holotype of the former ; H. subpallida sp. nov. ; H. kiiensis sp. nov. ; and Herdmania sp. (=H. momus, sensu Millar 1975). A tabular comparison of these Japanese species, together with H. curvata Kott, 1952 (=H. contorta Monniot, 1992, syn. nov.), H. inflata (Van Name, 1918), and H. polyducta Monniot and Monniot, 1989, is compiled on the basis of a reexamination of the name-bearing types or paratypes of these latter species. Specific delimitations are based largely on the detailed structure of the gonads.
The Asian goby genera Chaenogobius Gill and Gymnogobius Gill, the two Western North Pacific representatives of the Chasmichthys Group, are revised. A key is provided for each genus. Chaenogobius includes two species found in shallow marine waters along the coasts of Japan and the Korean Peninsula. Gymnogobius includes thirteen species found in shallow marine, brackish, and fresh waters throughout Japan, the Russian Far East, the Kuril Islands, the Korean Peninsula, and the Yellow Sea. A new species, Gymnogobius opperiens, is described from Japan, the Russian Far East, and the Kuril Islands, and a species previously thought to be undescribed is recognized as Gymnogobius petschiliensis (Rendahl, 1924).