Cover legend: Tritonia diomedea (above left) and Hermissenda crassicornis (below right) are both nudibranchs (Mollusca, Gastropoda, Opisthobranchia). As pictured here, Tritonia is in the midst of a dorsal body flexion as part of its swimming behavior. Hermissenda is incapable of swimming in this manner. Although these species have homologous neurons, they differ in serotonergic neuromodulation, which may underlie the difference in behavior. For more information, see the article by Lillvis and Katz. (pages 2709–2717).