Abstract
The pharmacology of acetylcholine and 5-hydroxytryptamine on the rectums of Katelysia rhytiphora and K. scalarina was found to be similar, in general, to that of other bivalves. However, while acetylcholine causes a fast twitch in the rectum of K. rhytiphora, the activity and tone of the K. scalarina rectum are depressed except at high concentrations of the drug. The two species can be distinguished by these responses, and, therefore, these rectums are useful experimental objects for studying the physiology of molluscan visceral muscle.