Fig. 8. Four stages in the formation and shedding of a ring vortex during a single half-stroke of the tail fin. The larva is viewed from above and is swimming directly towards the bottom of the page along a path coinciding with a streamer of milk. The drawings outline the main features of this sequence. At the 0 ms stage (A), the tail fin is about to begin a right-to-left swing and its right surface is in contact with the streamer. As the fin moves to the animal’s left (B), it draws a current of water in its wake which, by the 40 ms stage (C), has rolled up into a ring vortex which is about to be shed. The vortex propagates to the side and to the rear in the direction of the arrow shown in the 200 ms drawing.