As the _Sternoptyx_ is a delicate little creature, and quite defenceless, its illumination must be a fatal gift. This is equally true of the _Argyropelecus hemigymnus_ (Plate X., Fig. 3), a curiously formed fish,--deep in the body, tapering off suddenly to the tail, as if a piece had been bitten out by some large fish. Referring to the figure, it will be seen that the luminous organs are grouped; four being at the side of the tail, six midway between it and the line of the dorsal fin, and many others around the edge of the ventral surface,--one hundred and six in all: so that if all these plates are luminous, the _Argyropelecus_ must present a dazzling sight as it darts along in the blue waters of the Mediterranean, where it has been most commonly observed. Concerning the functions of these organs, there is still much controversy. The opinions of Ussow, Leydig, and others will be found in their papers referred to in the bibliography; and, as the question is thoroughly a technical one, its further discussion is omitted. As early as 1865 Professor Leuckart suggested that the curious plates (Plate X., Fig. 4) were organs of sight, or accessory eyes. In 1879 Dr. M. Ussow, of the University of St. Petersburg, gave the world an account of his researches upon the plates of the genera _Astronesthes_, _Stomias_ (Plate XX.), _Chauliodus_ (Plate XXI., Fig. 4), _Scopelus_ (Plate X., Fig. 1), _Maurolicus_, _Gonostoma_, and _Argyopelecus_, small fishes, most of which were found in the Mediterranean. This was followed by similar investigations by Dr. Leydig of Bonn, and Dr. Günther. A well-known phosphorescent fish is seen in _Scopelus_, which bears upon its sides and various parts of the body numbers of spots (Plate X., Fig. 1), which, if all luminous, mark it as one of the most brilliant of the light-givers. The appearance of these organs in reflected light is shown in Plate X., Fig. 2. The snake-like _Stomiasboa_ (Plate XX.), from a depth of twenty-seven hundred feet, is perhaps the most hideous of the light-givers; its large mouth and ferocious teeth giving it a bull-dog aspect, which in a large fish would make a veritable dragon. But _Stomias_ is not over twelve or fifteen inches in length, though quite large enough to terrify the smaller fry. The specimen figured was taken in the Gulf of Gascony by the naturalists of the “Talisman,” from its home, a mile and a quarter beneath the surface. The sides of the body are provided with a double row of luminous disks, which, according to M. Filhol, “cause the fish to be surrounded by a brilliant luminous aureola.” [Illustration: PLATE XVIII. CHIASMODUS. SUN-FISH. PLAGIODUS. HARPODON. BERYX.]