These organs have been regarded by many as luminous organs. A single glance shows that the body and lateral walls of the disk shine with a silvery and golden luster, but not different from that of the background of a fish's eye when viewed before a screen. More striking is the appearance in the case of the larger organs of the head in certain species, which are preeminently marked by it as a luminous apparatus. But, if the sole object of the apparatus were the collection and reflection of the light which fell upon the fish, its complicated structure in other respects, and its innervation, would be superfluous and still more incapable of explanation. We have, however, an observation that seems to show that these organs not only collect light, but are also really phosphorescent. The distinguished naturalist of the Challenger Expedition, Willemoes-Suhm, now deceased, saw Scopelius phosphorescent in the night, of which he says: "One of them hung in the net like a shining star as it came out of the darkness. Possibly the seat of the light is in the peculiar side organs, and it may be that this phosphorescence is the only source of light in the great depths of the sea." The thought that in the dark abysses of the deep sea every animal carries its lantern as the miner carries his lamp on his head, is a very fascinating one; and, indeed, Herr Willemoes-Suhm observed several other fishes that were provided on the smooth head and on the head-beard with "a remarkably large sense-organ." Valenciennes has also remarked of the genus Hemirarnphus that it bears a strongly glittering phosphorescent pustule on the tip of its tail. Although the majority of these animals have never been observed in a living condition, we might easily agree to the opinion that the organs
Fig. 6.—Scopelus Rafinesquii, twice the natural size. Two luminous organs in the ocular region.
of all three categories serve as a more or less perfect illuminating apparatus; and, if we compare Professor Leydig's sections of them, this opinion, which is only very apparent at the first view, becomes extremely probable. Especially does the section of the eye-like organ of Argyropelecus and Ichthyococcus resemble the illuminating parts of a projection-apparatus. If we conceive the granular spot in the center, into which the nerves enter, as the source of light standing in the middle of the apparatus, there are likewise behind this the concave reflector, and in front of it the diaphragm through which the concentrated cone of rays is thrown outward under a strong refraction.
