THE PHYSIOLOGY OF DEEP-SEA LIFE. 307 



fathoms in depth, must habitually move, when they come to the 

 surface, from regions of darkness through tracts gradually be- 

 coming lighter, until they strike the top of the water, which 

 they find flooded with white light. The range of the nature 

 of sunlight which they pass through is far greater than any we 

 can experience in going from the level of the sea to the high- 

 est mountain summits. But the phenomenon of the green Hght 

 of the sea, gradually replaced near the surface by white light, 

 must be similar to the one so well described by Langley, when 

 he ascended Mount Whitney to a height so much nearer the 

 upper limits of the atmosphere that the color of the sun ap- 

 peared blue, owing to the disproportionate amount of blue and 

 violet tints, which have been mainly resorbed selectively by the 

 upper layers of the atmosphere, allowing only the white rays 

 to reach us. 



In discussing the question of the penetration of light to 

 great depths, we shoidd not forget, on the one hand, that blind 

 Crustacea and other marine invertebrates without eyes, or with 

 rudimentary organs of vision, have been dredged from a depth 

 of less than 200 fathoms, and, on the other, that the fauna, 

 as a whole, is not blind as in caves, but that by far the majority 

 of the animals living at a depth of about 2,000 fathoms have eyes 

 either like their allies in shallower water, or else rudimen- 

 tary, or sometimes very large, as in the huge eyes developed out 

 of all proportion in some of the abyssal Crustacea and fishes, 

 and undoubtedly adapted to make the most of the little Ught 

 existing in deep water. These, as well as the appendages of 

 such deep-sea fishes as the Lophioids, intended to attract prey, 

 seem to tell us something of the exceedingly faint light, or of 

 the actinic rays which alone may reach them through the ap- 

 parently transparent water.^ 



The eyes of the deep-water fishes, Crustacea, cephalopods, 

 gasteropods, anneHds, echinoderms, and ccelenterates, are in 

 general identical in structure with those of the same classes liv- 

 ing upon the coasts in shallow water. We know little regard- 



1 At a distance from the shore, out of shelf by the action of the waves and cur- 

 reach of all the sediment brought down rents, sea-water is remarkably clear, and 

 by rivers, or washed from the continental free from impurities. 



