Details from Youtube:
During experiments on the axons of the Woods Hole squid (loligo pealei), we tested our cockroach leg stimulus protocol on the squid's chromatophores. The results were both interesting and beautiful. The video is a view through an 8x microscope zoomed in on the dorsal side of the caudal fin of the squid. We used a suction electrode to stimulate the fin nerve. Chromatophores are pigmeted cells that come in 3 colors: Brown, Red, and Yellow. Each chromatophore is lined with up to 16 muscles that contract to reveal their color.