What is the source of the calcium that activates contraction of barnacle muscles under physiological conditions?
D000818:Animals; D002118:Calcium / Q000502:physiology*; D008274:Magnesium / Q000502:physiology; D008564:Membrane Potentials; D009119:Muscle Contraction; D009132:Muscles / Q000502:physiology; D011188:Potassium / Q000502:physiology; D001468:Thoracica / Q000502:physiology
D008274:Magnesium; D011188:Potassium; D002118:Calcium