The nematode C. elegans shown here expresses Halorhodopsin in its motor neurons (Punc-17::Halo::CFP ). When green laser light shines on a small anterior region of the worm's ventral nerve cord, it inhibits those motor neurons and the worm's tail becomes paralyzed. Note how the worm's tail retains its shape when the light is applied and it does not relax to neutral. The laser is on when the words "DLP On" appear.
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.