A) The squid’s light organ is the anatomical structure that maintains the bioluminescent symbiont V. fischeri and modulates light output. The organ is located underneath the mantle of the squid, just atop the funnel: a structure used to move water into and out of the mantle cavity. The immature light organ (boxed in red, and enlarged at right) has bilateral ciliated fields and appendages. At the base of the appendages are pores, leading into the crypts of the light organ. B) A cross-section of the light organ, boxed in blue in (A), shows that the symbiont (teal) is maintained in extracellular crypts lined with a polarized epithelium that is photoreceptive. Structures surrounding the crypts, such as the reflector (indicated in green dashed lines), ink sac, and lens manipulate the light produced by the symbiont for host behaviors.