Fig. 1. Schematic picture of a longitudinal section through a ventral photophore from krill. Light is produced in the lantern (La) that is made up by processes from B-cells (possibly photocytes; B) and refractive rods. Light produced in the lantern is reflected against the inside of the organ wall (reflector; R) and passes through a lens (Le) before leaving the photophore. Apart from B-cells, three other cell types are present in the organ – large A-cells (A) and smaller C-cells (C) and D-cells (D). On both sides of the lens, photophore vessels (V) and nerves (N) enter the organ. Capillaries branch off from the arteries and pass both D- and C-cells before they empty in the lantern. Nerves follow the capillaries and end at a sphincter-like structure at the base of the C-cells. Modified from Herring and Locket (Herring and Locket, 1978) with permission.