184 DEEP-SEA FISH. 



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 

 endeavouring 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.* 



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 cannot 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 



♦ Guather, " Introduction to the Study of Fishes." 



