A note on the evolution of the transducer mechanism of the vertebrate retinal rod.

The Ca2+-gating mechanism that is the key component of membrane excitability in paramecium has recently been localized on the cilia of this animal. Such a finding shows how the change in Ca2+ permeability (which is the probable consequence of the photoisomerization of rhodopsin) of the discs of the outer segments of the retinal rods could have evolved.