The structures of the alimentary canal of Pteropoda, including the oesophagus, crop, gizzard and stomach with a caecum, resemble to those of Dolabella auricularia. The dichotomous branching of the tubules of Pteropoda was akin to that of some Gastropoda and Cephalopoda, elucidated by my previous works. On the other hand, the small dimension of the tubules in cross section may be supposed to correlate with the intracellular digestion, the peculiar habit of plankton feeder as in the case of Pelecypoda.

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Both forms of L. onychia were collected in Chikubu-jima and Kitayamada in Lake Biwa respectively in April, 1954. There are morphological differences between these 2 forms in the shell, buccal mass, salivary gland, radula, caecum, hermaphrodite gland and duct, egg-membrane gland, prostate gland, male copulatory organ, its retractor muscles and pedal nervous system. The eggs of both forms could not be examined yet. On mature consideration of these characteristic features of both forms, Omia seems to be assigned to a subgenus of Lymnaea. These differences between both forms may be insufficient to separate them into 2 distinct species but, at least, it may be sure that ecological forms. The more attentive ecological and the chromosomal studies of them may throw light on the specific distinction and the determination of systematic position of L. onychia.

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