The tidal rhythms for changes in morphology, length, dry weight and protein content of the crystalline style are studied in two venerid clams,Meretrix meretrix andKatelysia opima and in a donacid clam,Donax cuneatus. In the two venerid clams the style is housed in a sac conjoined with midgut and there is a tidal rhythm of feeding and extracellular digestion involving a partial dissolution of the style in the ebb tide and reformation in flood tide. But in the donacid clam where the style sac is completely separated from midgut, no detectable changes in the style parameters are recorded during a tidal cycle. However, the rhythmicity in feeding activity is indicated, by the presence of ‘mudcap’ and food particles in the style during flood tide and their subsequent absence during ebb tide. Hence the tidal rhythm of feeding and digestion inDonax cuneatus may be without corresponding changes in the crystalline style. Also this study throws light on the mode of style dissolution.