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The genusSemicytherurabelongs to the family Cytheruridae, and was distinguished fromCytheruraon the basis of carapace features. Species ofSemicytherurafrom Japan and adjacent seas can be divided into two groups. One is represented bySemicytherura miurensisHanai, 1957, characterized by a thin, oval carapace covered with fine reticulation. The other is represented bySemicytherura henryhoweiHanai&Ikeya, 1977, characterized by a thick sub-rectangular carapace in lateral view.Semicytherura henryhowei, which is distributed from Hokkaido to Okinawa in Japan, has been regarded as having several morphotypes distinguishable on outline and reticulation of carapace. However, as a result of detailed observations on the copulatory organ, carapace outline and distributional pattern of pore systems, remarkable differences are shown to exist between the two most frequently occurring morphotypes. In order to recognizeS. henryhowei sensu stricto, the carapace of the holotype was re-examined. Consequently, neither of the two morphotypes are considered to belong toS. henryhoweidue to differences of carapace outline and distribution of pore systems. The two morphotypes are here regarded as independent taxa, described as new:S. kazahanan. sp. andS. sasameyukin. sp. The geographical distributions of the two new species overlap, but their micro-habitats differ from each other; the former lives on calcareous algae on rocky shores, the latter lives on silty sand bottom within the inner bay. A third new species,S. slipperisp. nov., is also described. In view of their present geographical distributions and fossil records, the origin of this group of species would appear to be the Japanese islands or adjacent areas in and after the Miocene. This group then migrated to the Arctic Ocean and East Pacific Ocean during or before the middle Pliocene. [ABSTRACT FROM AUTHOR]

The genus Semicytherura develops a broad thin calcareous lamella inside the adult carapace, and this feature is one of the characters which define this taxon. However, neither anatomical nor histological studies have so far been done on this character. In earlier works, the lamella was regarded as an extension of the marginal infold, i.e. the "calcified inner lamella cuticle." In this study scanning and transmission electron microscope observations have clarified that the lamella is a different structure from the outer lamella cuticle, and that the epidermis does not exist inside the lamella. The lamella is called the "prismatic layer," because of its calcareous crystal form. Development of this layer causes a restriction of the epidermis, which has a function in respiration and metabolism. Species of this taxon are thought to support their respiratory and metabolic systems by their small body size and low energy of locomotion, or through some specialized structures in the epidermal layers. [ABSTRACT FROM AUTHOR]

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