The taxonomy on two Recent species of the genus Orectospira and their bathymetrical distributions were studied on a precise data of depths and locations off the coast of Japan. Orectospira Dall, 1925, a small gastropod genus of family Orectospiridae, is known to have a subtle difference between adult and juvenile shells. Through the examination of shell-height/shell-width ratio and characteristics of growth lirae, unbilicus and siphonal canal among three species described so far, it became clear that only O. tectiformis (Watson, 1880) and O. shikoensis (Yokoyama, 1928) were valid Recent Japanese species belonging to the genus Orectospira and that O. babelica (Dall, 1907) is a synonym of O. tectiformis. This synonymization was also supported by bathymetric distributions of above species, as both O. tectiformis and O. babelica were found in the same depths from 700 to 1100 m. On the other hand, O. shikoensis was collected from depths between 150 and 800 m, especially around 300 to 600 m, and consequently a clear boundary of the bathymetrical distribution between O. tectiformis and O. shikoensis exists at depths between 700 and 800 m.

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Conus swainsoni n.sp., a member of the C. planorbis group, is described from New Caledonia. It inhabits coarse muddy sand with detritus and calcareous algae in about 20 m and occurs sympatrically with C. connectens A. Adams, 1855. Although the species had already been named C. pulchellus by Swainson (1822), it has no available name, and Swainson's type cannot be traced. Rather than establishing a nomen novum which is not stabilized by a type specimen, an entirely new description is made, including a biometrical comparison with the most closely related species C. connectens.

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The gonad and gametogenesis in a common species of chiton, Acanthopleura (=Liolophura) japonica (Lischke), have been studied by paraplast section using both Azan and Feulgen staining methods. The gonad showing a single lobulated structure is located along the dorsal aorta under the shell valves. In both ovary and testis, flattened ovarian and testicular folds are elongated from the germinal epithelium onto the dorsal aorta. Oocytes in three different developmental stages can be distinguished in the ovary ; previtellogenic, vitellogenic and full-grown oocytes. Oocytes of the former two stages are attached to the ovarian folds, while oocytes in the last stage are scattered in the lumen of the ovary. The breakdown of oocytes is often found to occur and a number of them will disappear before maturation. In the testicular folds, the spermatogonia layer is nearest to the basal membrane, and next, are layers of primary spermatocytes and spermatids respectively. Spermiogenesis occurs in the lumen of the testis.

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Among pectinids presenting a variety of sexual expressions, the sea scallop Placopecten magellanicus has long been considered as strictly gonochoric. Based on observations made on several cases of hermaphroditism found in 1984, a possible protandric sexuality with an early sexual inversion for some of the individuals is hypothesized. The wide variety of histological images observed might be an indication of the delicate equilibrium of sex determination in the Phylum Mollusca.

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The number of chromosomes and karyotypes in four species of the genus Semisulcospira, S. libertina, S. dolorosa, S. trachea and S. nakasekoae, were analyzed by air-dry method using gill tissue treated by colcichine, either in vivo or in vitro. Two species, S. libertina and S. trachea, which were showed to have 18 pairs of chromosomes by Burch (1968), had 18 pairs of chromosomes, together with S. dolorosa. These three species showed no differences even in karyotypes, consisting of 16 metacentric, 14 submetacentric, 4 acrocentric and 2 terocentric chromosomes. While the chromosome number of S. nakasekoae distributed only in the lake Biwa was 19 pairs of chromosomes, 2n=38, consisting of 6 metacentric, 24 submetacentric and 2 telocentric chromosomes. This much differences from what reported by Burch, 2n=26, suggest that a divergence in karyotypes may exist within the species living in the Lake Biwa.

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