Cover legend: The head of amphioxus (Branchiostoma floridae). According to molecular phylogeny, amphioxus is the most basal extant chordate. In the rostral end of its neural tube, two classes of microvillar photoreceptors are found. These cells sense light using the photopigment melanopsin, like circadian receptors of higher vertebrates, and generate an electrical response by mechanisms akin to those of “rhabdomeric” visual cells of mollusks and arthropods. For more information, see the article by Ferrer et al. (pages 17977–17987).