This image shows two nudibranchs (sea slugs) engaged in their characteristic swimming behavior, which consists of a series of laterally directed body flexions. In Melibe leonina (top), swim interneuron 1 (Si1) is part of the central pattern generator (CPG) for swimming; it fires rhythmic bursts of action potentials during a swim bout. Surprisingly, the homolog of Si1 in Dendronotus iris (bottom), serves as a neuromodulatory command neuron for a homologous swimming behavior; it fired irregularly throughout the swim motor pattern. For more information, see the article by Sakurai and Katz. (pages 6460–6471).