The nematode C. elegans shown here expresses Halorhodopsin in its muscle cells (Pmyo-3::Halo::CFP ). When green laser light shines on the worm's neck region the muscles in that area are induced to relax. Note also how the worm's tail relaxes to a neutral position even as the worm's head continues to make bending waves.
This is an example of the CoLBeRT system from the Samuel Lab at Harvard University from the paper Leifer et al, "Optogenetic manipulation of neural circuits in freely moving Caenorhabditis elegans," Nature Methods, in press, 2011.
See colbert.physics.harvard.edu for more.