Decapod Crustacea of the North Atlantic. 195 



These facts and the comparison of the eyes and the colour 

 of the abyssal species with tlie blind and colourless cave- 

 dwelling Crustaceans certainly indicate some difference in the 

 conditions as to light in caverns and in the abysses of the 

 ocean, and make it appear probable, in spite of the objections 

 of the physicists, that some kind of luminous vibrations do 

 penetrate to depths exceeding even 201)0 fathoms. The fact 

 that, excluding shallow-water species, there is no definite rela- 

 tion between the amount of the modification of the eyes and 

 the depth which the species inhabit, many of the species with 

 the most highly modified eyes being inhabitants of much less 

 than 1000 fathoms, might at first be thought antagonistic to 

 this view. But when we consider how vastly greater the 

 purity of the water must be iu the deep ocean far from land 

 than in the comparatively shallow waters near the borders of 

 the continents, and how much more transparent the waters of 

 the ocean iibysses than the surface waters above, we can 

 readily understand that there may usually be as much liglit 

 at 20O0 fathoms in mid-ocean as at 500, or even at 200, near a 

 continental border. These considerations also explain how 

 the eyes of specimens of species like Paropagurus ptHosimanus, 

 coming from 2220 fathoms, are not perceptibly different from 

 the eyes of specimens from 250 fathoms. 



Although some abyssal species do have well-developed 

 black eyes, there can be no question that there is a tendency 

 towards very radical modification or obliteration of the normal 

 visual organs in species inhabiting deep water. The simplest 

 and most direct form of this tendency is shown in the gradual 

 reduction in the number of the visual elements, resulting in 

 the obsolescence and in some cases in final obliteration of the 

 eye. The stages of such a process are well represented even 

 among the adults of living species. The abyssal species with 

 black eyes referred to in a previous paragraph contain the first 

 part of such a series, beginning with species like Geryvn 

 quinq^uedens and Lithodes Agassizii and ending with Ethusina 

 abyssicola^ iu which there are only a few visual elements at 

 the tips of the immobile eyestalks. A still later stage is re- 

 presented by A. Milne-Edwards's genus Cymonomus, in which 

 the eyestalks are immobile spiny rods . tapering to obtuse 

 points, without visual elements or even (according to the 

 description) a cornea. Cymonomus is not known to be an 

 abyssal genus, neither of the species having been recorded 

 from raucJi below 700 fathoms, and is a good example of the 

 fact already mentioned that many of the species with the most 

 highly modified eyes are inhabitants of comparatively shallow 

 water. There are, however, several cases of closely allied 



