Fig. 1. Sketch of a cross section in the medio-frontal plane of the wake behind a steady undulatory swimmer. (A) Double row of single vortices as observed in eel, bream, trout and mullet, which in three dimensions constitute a chain of vortex rings (Blickhan et al., 1992). (B) Double row of double vortices as observed in zebra danio, water snake and Kuhli leach. The circles indicate shed vortices, with arrowheads indicating the rotational sense. The arrows indicate the jet flows.