It is rare to be able to explain the behaviour of a whole animal at the level of the properties and connections of characterized CNS neurones. In a marine mollusc, Clione, and a lower vertebrate embryo, Xenopus, it is possible to make intracellular recordings during fictive swimming behaviour. This has allowed us to analyse the operation of two central pattern generators (CPGs) at the cellular level. Although the timeframes over which the two CPGs operate are different, there are significant similarities in their patterns of neural output. A detailed analysis of the neural networks involved reveals that the swimming CPGs of Clione and Xenopus have several common operating principles, which suggests that common mechanisms have evolved to perform similar tasks, despite differences in neuronal 'hardware'.