Photoreceptors in primitive chordates: fine structure, hyperpolarizing receptor potentials, and evolution

Two species of primitive chordates have hyperpolarizing photoreceptor potentials, as vertebrates do. In Salpa the photoreceptive membrane is composed of microvilli, whereas in Amaroucium it is modified from cilia. There appears to be no functional correlation between fine structure of photoreceptive membrane and polarity of response to light.