Embryonic development of the freshwater planorbid snail, Helisoma duryi, reared under laboratory controlled conditions was described in detail. The morphological characters and the time course of embryonic stages are presented. H. duryi generally lay egg masses at night or in the early morning hours. The egg type of H. duryi is isolecithal, and the cleavage is spiral holoblastic. After early cleavages followed by the appearance of the blastula, gastrula, and larval (trochophore, veliger, veliconch) stages, the fully developed young snail start hatched on the eighth day. Many unique developmental features in studied snail were observed and discussed.
Key words: Freshwater snail, Helisoma duryi, Planorbidae, Embryonic development, cleavage.
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