The nematode C. elegans shown here expresses Halorhodopsin in its motor neurons (Punc-17::Halo::CFP ). When green laser light shines on its ventral nerve cord it it inhibits the motor neurons and the worm paralyzes. When the laser light illuminates the dorsal nerve cord no effect is observed. 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 http://colbert.physics.harvard.edu for more.