The responses of Aurelia medusae to pharmacological agents and ionic variation were classified into four response types: Type I, no response; Type II, inhibition of pacemaker activity; Type III, inhibition of both pacemakers and swimming muscles; and Type IV, increase in pacemaker output. 2. The swimming pacemakers of Aurelia medusae become hyperactive in Mg+2-free solutions (Type IV). This response appears to be general in swimming scyphozoa. 3. The response pattern to pharmacologically-active compounds indicates that the coelenterate neuromuscular system is quite different than those in other phyla. In fact, the response spectrum is not consistent within the Cnidaria. 4. Similarly, the responses of adult medusae to ionic variation show no consistent pattern within various scyphomedusae. 5. Test solutions from each response type established with medusae were selected and tested on the scyphistoma and strobila stages. The comparison of the responses to the test solutions between the medusa, scyphistoma, and strobila showed that the neuromuscular systems are physiologically different. The strobila, specificially the ephyra, is a mixture of both polypoid and medusoid response types. The strobila, therefore, is physiologically an intermediate stage in the development of the adult medusa.
