Crawling behavior in the nearly-intact leech and its dependence on descending signals from the cephalic ganglion. A, Schematic drawing of the nearly-intact leech preparation. A bowtie-shaped well was placed across the connectives between M2-M3. The well was sequentially filled with saline, isotonic sucrose, and then saline to restore neural transmission. B, Schematic drawing of overt crawling behavior in nearly-intact leech preparations when descending signals were not blocked. Numbers i.-iii. indicate the contraction phase and iv.-vi. the elongation phase of one overt crawl cycle. C, Number of spontaneous crawl cycles observed for 8 animals during a 5-min interval for each condition (i.e., Pre-Block, Blocked, Post-Block). D, Number of trials (out of 5) that crawling was elicited in 8 animals after a gentle hyperelongation of the body; same three conditions were tested. C,D, Black bars represent shallow water (< 1 mm depth) and grey bars represent deep water (10 mm depth). ** = p < 0.01. Error bars are ± SEM.