“The only part of the under surface of the animal which was free from luminosity was the black collar around the throat; and while the inferior surface of the pectoral, anal, and caudal fins shone with splendor, their superior surface (including the upper lobe of the tail-fin) was in darkness; as also were the dorsal fins, back, and summit of the head. I am inclined to believe that the luminous power of this shark resides in a peculiar secretion from the skin. It was my first impression that the fish had accidentally contracted some phosphorescent matter from the sea, or from the net in which it was captured; but the most rigid investigation did not confirm this suspicion, while the uniformity with which the luminous gleam occupied certain portions of the body and fins, its permanence during life, and decline and cessation upon the approach and occurrence of death, did not leave a doubt in my mind that it was a vital principle, essential to the economy of the animal. The small size of the fins would appear to denote that this fish is not active in swimming; and, since it is highly predaceous, and evidently of nocturnal habits, we may, perhaps, indulge in the hypothesis that the phosphorescent power it possesses is of use to attract its prey, upon the same principle as the Polynesian Islanders and others employ torches in night-fishing.” [Illustration: PLATE XIX. LUMINOUS FISH. (_Eustomias obscurus._) From depth of 8,100 feet.] CHAPTER XIII. FINNY LIGHT-BEARERS--(SURFACE FORMS). On calm nights the splash of the oars and the fall of spray from the bow of the boat startle many fishes resting at or near the surface, which dart away like comets, leaving a blaze of light behind, and giving the impression that they are light-givers or phosphorescent. This does not always follow; as, while many possessors of luminous spots undoubtedly approach the surface at night, as _Scopelus_ (Plate X., Fig. 1), many owe their brilliant appearance to the luminosity of the medium in which they swim; in other words, the vigorous motion of their fins produces the same effect and result that is attained by darting the hand through water bearing phosphorescent animals. If such a display is produced by one fish, we may well imagine that a school moving rapidly would create a light of considerable intensity. Drifting over a school of menhaden, and peering down among them, each fish seems outlined in a golden halo; while coruscations of light appear to flash from the fishes as they move along, the presence of the school being indicated upon the water by a pale luminous spot.