=Photophores or Luminous Organs.=--Many fishes, chiefly of the deep seas, develop organs for producing light. These are known as luminous organs, phosphorescent organs, or photophores. These are independently developed in four entirely unrelated groups of fishes. This difference in origin is accompanied by corresponding difference in structure. The best-known type is found in the Iniomi, including the lantern-fishes and their many relatives. These may have luminous spots, differentiated areas round or oblong which shine star-like in the dark. These are usually symmetrically placed on the sides of the body. They may have also luminous glands or diffuse areas which are luminous, but which do not show the specialized structure of the phosphorescent spots. These glands of similar nature to the spots are mostly on the head or tail. In one genus, _Æthoprora_, the luminous snout is compared to the headlight of an engine. [Illustration: FIG. 142.--Headlight Fish, _Æthoprora lucida_ Goode and Bean. Gulf Stream.] [Illustration: FIG. 143.--_Corynolophus reinhardti_ (Lütken), showing luminous bulb (modified after Lütken). Family _Ceratiidæ_. Deep sea off Greenland.] Entirely different are the photophores in the midshipman or singing-fish (_Porichthys_), a genus of toad-fishes or _Batrachoididæ_. This species lives near the shore and the luminous spots are outgrowths from pores of the lateral line. In one of the anglers (_Corynolophus reinhardti_) the complex bait is said to be luminous, and luminous areas are said to occur on the belly of a very small shark of the deep seas of Japan (_Etmopterus lucifer_). This phenomenon is now the subject of study by one of the numerous pupils of Dr. Mitsukuri. The structures in _Corynolophus_ are practically unknown. [Illustration: FIG. 144.--_Etmopterus lucifer_ Jordan and Snyder. Misaki, Japan.] =Photophores in Iniomous Fishes.=--In the _Iniomi_ the luminous organs have been the subject of an elaborate paper by Dr. R. von Lendenfeld (Deep-sea Fishes of the Challenger. Appendix B). These he divides into ocellar organs of regular form or luminous spots, and irregular glandular organs or luminous areas. The ocellar spots may be on the scales of the lateral line or on other definite areas. They may be raised above the surface or sunk below it. They may be simple, with or without black pigment, or they may have within them a reflecting surface. They are best shown in the _Myctophidæ_ and _Stomiatidæ_, but are found in numerous other families in nearly all soft-rayed fishes of the deep sea. The glandular areas may be placed on the lower jaw, on the barbels, under the gill cover, on the suborbital or preorbital, on the tail, or they may be irregularly scattered. Those about the eye have usually the reflecting membrane.