Slide showing photograph of 'The response of a spinal eel to a persistent stimulus applied to the tip of the tail.' Slide shows black and white photograph in 8 sections overlaid with diagram of the spinal eel's reaction to stimulus. Title below image reads 'Figure 21. The response of a spinal eel to a persistent stimulus applied to the tip of the tail. Note the formation and movement of active waves of contraction indicated by the symbols . [dot], x [cross], + [plus sign]. (From Gray 1936)'. Late 19th - early 20th century