Some considerations on the fine structure of rhabdomeric photoreceptors in the amphioxus, Branchiostoma lanceolatum (Cephalochordata).

This study confirms the close relationship of two of the photoreceptive cells of the amphioxus central nervous system, the Hesse and Joseph cells. Both cells have rhbdoms located at the cell surface and possess 9 + 0 cilia that are not related to the rhabdom structure. The rhabdom of Joseph cells is enclosed by thin glial lamellae containing gliofilaments, whereas that of Hesse cells is associated with a pigmentary cell, the two cell types forming an eyecup. The pigment cell is mostly filled of pigment granules and has a 9 + 2 cilium. A third photoreceptive cell of the lancelet, the lamellate cell, is of ciliary type. The significance of the presence of rhabdomeric and ciliary photoreceptors in the lancelet brain is discussed.