Fig. 1. Diagram of experimental setup. Two light tables are shown as yellow blocks on opposite sides of the aquarium (light yellow block in center) from the cameras. Camera 1 recorded the lateral view of the worm and camera 2 recorded the dorsal or ventral view. Schemes of the 2-D views of each camera are shown with corresponding axes, and the orientation of the worm in the crack is shown in the aquarium. Between the light table and the aquarium are a colored filter and a right-handed, circular polarizing filter with a left-handed, circular polarizing filter and another color filter between the aquarium and the camera. The circular polarizing filter is shown here as a linear polarizing filter with a quarter-wave retarder; actual filters combine the two components. Filters on the far side of the cameras completely covered the light tables with no other light passing through, and the filters on the camera side were attached to the lenses. Cameras were run from separate computers with LabView software. The defined coordinate system is used in all relevant figures.