Video S4 In the absence of all-trans retinal, no Ca2+ transient is induced under the blue light when ChR2 is expressed. Found at: doi:10.1371/journal.pgen.1001326.s013 (1.71 MB WMV) Video S5 Optical stimulation of the gubernacular-oblique muscles, which express ChR2, elicits Ca2+ transients in the spicule protractors in the presence of all-trans retinal. Found at: doi:10.1371/journal.pgen.1001326.s014 (0.83 MB WMV) Video S6 Optical stimulation of the gubernacular-oblique muscles, which express ChR2, does not elicit Ca2+ transients in the spicule protractors in the absence of all-trans retinal. Found at: doi:10.1371/journal.pgen.1001326.s015 (0.53 MB WMV) Video S7 Laser stimulation of the gubernacular erector induces Ca2+ transients in the spicule protractor-anal depressor muscles. Found at: doi:10.1371/journal.pgen.1001326.s016 (4.91 MB WMV) Video S8 Laser stimulation of a body wall muscle does not induce Ca2+ transients in the spicule protractor-anal depressor muscles. 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