Ussow[1] has more recently maintained that they are eyes, and Leydig considers them as organs which approach very nearly to true eyes ("welche wirklichen Sehorganen sehr nahe stehen"). Whatever doubt there may be whether they have any power of sight, there is no longer any question but that they are luminous, and they are especially developed in the fishes of the deep sea.
These are very peculiar. The abysses of the ocean are quite still, and black darkness reigns. The pressure of the water is also very great.
Hence the deep seas have a peculiar fauna of their own. Surface species could not generally bear the enormous pressure, and do not descend to any great depth. The true deep-sea forms are, however, as yet little known. They are but seldom seen, and when obtained are generally in a bad state of preservation. Their tissues seem to be unusually lax, and liable to destruction. Moreover, in every living organism, besides those usually present in the digestive organs, the blood and other fluids contain gases in solution. These, of course, expand when the pressure is diminished, and tend to rupture the tissues. The circumstances under which some deep-sea fish have occasionally been met with on the surface bears this out. They are generally found to have perished while endeavoring to swallow some prey not much smaller, or even in some cases larger, than themselves. What, then, has happened? During the struggle they were carried into an upper layer of water. Immediately the gases within them began to expand, and raised them higher; the process continued, and they were carried up more and more rapidly, until they reached the surface in a dying condition.[2]
It is, however, but rarely that deep-sea fish are found thus floating on the surface, and our knowledge of them is mainly derived from the dredge, and especially from the specimens thus obtained during the voyage of the Challenger.
In other respects, moreover, their conditions of life in the ocean-depths are very peculiar. The light of the sun can not penetrate beyond about two hundred fathoms; deeper than this, complete darkness prevails. Hence in many species the eyes have more or less completely disappeared. In others, on the contrary, they are well developed, and these may be said to be a light to themselves. In some species there are a number of luminous organs arranged within the area of, and in relation to, the muciferous system; while in others they are variously situated. These luminous organs were first mentioned by Cocco.[3] They have since been stud-
