One of the surprises of the early explorers of the depths of the ocean was that many of the tropical pelagic fishes had luminous organs; and for a long time it was believed that all of these came from the abysses where no particle of sunlight penetrated, and that thus the sty gian depths were illuminated. Later investiga tions have modified this view. The luminous fishes belong mainly to the families Stomiatidte, Sternoptychidm, Scopelidm and Ceratidx. All these are pelagic families and have species ranging from the surface to the deepest bot toms (see DEEP SP .A EXPLORATION). The exten sive explorations of the ocean depths carried on in the Valdivia, the Michael Sars and other vessels in recent times show that light-organs are "specially characteristic of fishes belonging to the upper 500 meters in warm oceanic waters.° This is true of the crustaceans and cephalopods, in the latter case squids, living at intermediate depths. The light-organs in these animals are distributed in various parts of the body, and are highly complicated, eye-like glands, each having not only a lens to increase its power, but a layer of black pigment in the rear, to act as a reflector. There is no doubt that this structure enables the fishes and squids to project the light in definite directions. The function and importance of these organs have been much discussed; and the purpose is still in question. "Is it in order to illuminate the surrounding water, to avoid foes, or to recog nize their own kind?° asks Hjort ((Depths of the Ocean,' 1912). "The answers,° he replies, °would probably tend to show that the many different kinds of light-organs serve different purposes.° Brauer finds that their position is
precisely the same in all individuals of any one . species, and concludes that they replace, in the darkness of the depths, the specific color-marks or urecognition-marks° of surface or terrestrial animals. As it seems certain that this emission of light is intended to be seen, the relation be tween them and the eyes of these fishes and other oceanic animals must be studied; but the data for this are defective and confusing. It appears that a remarkable coincidence exists between the development of light-organs and eyes in pelagic fishes. The Scopelidx, Sternop tychidm and Stomiatidm, which live above 500 meters, possess well-developed light-organs and eyes, while from 500 meters downward light organs and eyes both decrease in size. Along the sea-bottom, however, the fishes have only eyes and no light-organs.° The eyes of the bottom fishes (Macrurus) are large, as if to admit as much as possible of a scanty supply of light. In those depths all the invertebrate animals are aglow, clothed in a copious mucus that emits light; and it is believed that this il lumination is sufficient to enable the big large eyed bottom fishes to see what is about them. The situation is, nevertheless, full of puzzling contradictions. Consult books mentioned under DEEP SEA EXPLORATION; and for the most recent discussion of the matter consult Murray and Hjort's 'Depths of the Ocean) (London 1912).