Effects of Some Postulated Neurohumors on Rhythmicity of the Isolated Cloaca of a Holothurian

The cloacal pumping rhythms of holothurians have been of interest to a number of investigators attracted by the problem of the relation to oxygen consumption of movements of the entire active system comprising anal sphincter, cloaca, and respiratory trees (e.g., Newell and Courtney 1965). Relatively fewer investigators have been concerned with problems of endogenous rhythmicity or of transmission of excitation in the cloaca. Welsh (1966) has implied that the diffuse nature of the nervous system of echinoderms is the obstacle that has led to the indirect nature of much of the evidence for functioning echinoderm neurohumors. Wyman and Lutz (1930) concluded that speculation based on the antagonistic action of drugs was a futile approach to neuromuscular organization in holothurians. However, previous reports dealt with the direct action of drugs on spontaneous activity of the cloaca. The cloaca has a rather complex response to electrical stimulation, and it may be that some advance in understanding the significance of the antagonistic action of drugs can be obtained from experiments providing information about the action of postulated neurohumors and neurohumoral