ALT On the top left is a scanning electron microscopy image of a worm with a 50 micron scale bar in the bottom corner. Next to it is a photograph of a worm in a slice of fruit. Below is a schematic of the imaging system for ratiometric calcium imaging. Worms are shown swimming between two coverslips and illuminated in epifluorescent mode with a multi-color LED source, shown as an aquamarine line that travels through a 10X objective lens. The output green and red channels are split with a beam splitter and sent to two halves of a single camera sensor. On the far right are five false color images of C. elegans showing calcium-sensitive GCaMP3 fluorescence signals in cyan and the calcium-insensitive RFP reference in red. The largest image on the left labels the head and tail, two points of Kmax, and active regions along its side that are colored cyan. On the right are four images showing a time series from t=0 to 0.9 seconds of increasing calcium signal as a head bend is initiated.