In the jellyfish Aglantha digitale two forms of swimming arise from two separate propagating axonal impulses: a fast, overshooting action potential that depends on TTX-resistant Na⁺ channels, and a lowamplitude spike that depends on T-type Ca²⁺ channels. While the Na⁺ action potential is propagated simply and without distortion, the shape of the Ca²⁺ spike depends on the past history of the axon; it is processed as well as propagated. Patch- and voltage-clamp experiments show how three classes of K⁺ channels contribute to this apparently unique system. A dual Na⁺/Ca²⁺ impulse mechanism may increase the bandwidth of an axonal line of communication but it also places restrictions on the form of the synaptic input needed for spike initiation.