The hexactinellid sponge Rhabdocalyptus dawsoni is capable of arresting its exhalant water current in response to mechanical and electrical stimuli. The arrest is coordinated by a conduction system with a precise threshold of excitability and a chronaxie of 38 milliseconds. The response is propagated throughout the sponge at a mean velocity of 0.22 centimeter per second, and conduction is unpolarized.