Abstract
Hiatella orientalis is a small bivalve nearly allied to Hiatella (Saxicava) arctica. It is found in shallow waters from the central to the southern part of Japan. The breeding covers rather the cold season, extending from December to May. Sexes are separated, testis is milky white in colour and the ovary generally yellowish brown. Freshly shed ovum is ca. 60μ in diameter with a gelatinous coating ca. 18μ thick and the germinal vesicle is intact. Embryo produced by external fertilization enters into the typical pelagic life. In the early development there protrude no polar lobes. The veliger larva is furnished with distinct apical cilia. Fig. 16 shows fairly grown veliger ca. 222×194μ in size, which the posterior margin of the shell is more swollen, compared with to the anterior. The shell is greenish yellow in colour except for the dark blue at the umbonal region. Full grown larva just prior to spatting is about 379μ in length and 357μ in height.
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