[gchamonlive]

I'm not sure that's correct for very basic multicell organisms. I'm no biologist, but I believe primitive systems wouldn't need these types of advanced pathways when light sensitive cells could just produce signaling compounds to drive the behaviour of the rest of the organism without necessarily requiring a centralised nervous system, akin to how single cell organisms orient themselves in their environment.

[jhedwards]

This is correct. Rhodopsin, for example, is a G-protein coupled receptor, which means that activation of the receptor triggers G-proteins inside the cell which can activate or inhibit any number of cellular processes.