102 THE POPULAR SCIENCE MONTHLY. 



Ussow* has mofe 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. Sur- 

 face 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. More- 

 over, 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 dimin- 

 ished, 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 hap- 

 pened ? During the struggle they were carried into an upf)er layer 

 of water. Immediately the gases within them began to expand, 

 and raised them higher ; the process continued, and they were car- 

 ried up more and more rapidly, until they reached the surface in 

 a dying condition, f 



It is, however, but rarely that deep-sea fish are found thus float- 

 ing 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 mucif erous system ; 

 while in others they are variously situated. These luminous or- 

 gans were first mentioned by Cocco. J They have since been stud- 



* " Ueber den Bau der sog. augenahnlichen Flecken einiger Knochenfische," " Bull. Soc 

 Imp, Moscow," 1879. 



f Gunther, " Introduction to the Study of Fishes." 

 X "Nuovi Ann. dei Sci. Nat.," 1838. 



