MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 135 



species of Glyphocrangou and of Benthonectes are good examples of highly developed eyes of this 

 class. In many cases the presence of light- colored pigment is accompanied with reduction in the 

 number of visual elements precisely as in black eyes, Parapasiphae sulcatifrons, P. cristata, Acan- 

 thephyra microphthahna, and the species of Hymenodora being good examples. 



" In other cases there are apparently radical modifications in the structural elements of the 

 eye without manifest obsolescence. The large and highly developed but very short-stalked eyes of 

 the species of Glyphocrangou, apparently specialized for use in deep water, probably represent 

 one of the earlier stages of a transformation which results finally in the obliteration of the visual 

 elements of the normal compound eye and the substitution of an essentially different sensory 

 structure. In Pontophilus abyssi the transformation has gone further; the eyes, though fully as 

 large as in the allied shallow- water species, are nearly colorless, not very distinctly facetted, and 

 have probably begun to lose the normal visual elements over a portion of the surface. In the eyes 

 of several of the species of Thunidopsis the normal visual elements have entirely disappeared, and 

 there is an expanded transparent cornea, backed by whitish pigment and nervous elements of 

 some kind. I am well aware that there is as yet no conclusive evidence that these colorless eyes 

 are anything more than the functionless remnants of post embryonic or inherited organs; but the 

 fact that in some species they are as large as the normal eyes of allied shallow-water forms is 

 certainly a strong argument against this view. In the species of Pentacheles there is still better 

 evidence that the eyes are functionless; for, although they have retreated beneath the front of the 

 carapace, they are still exposed above by the formation of a deep sinus in the margin, and the 

 ocular lobe itself has thrown off a process which is exposed in a spinal sinus in the ventral mar- 

 gin. It is easy to conceive how these highly modified eyes of Pentacheles may have been derived 

 from eyes like those of the species of Glyphocrangon and PonthopMlus abyssi through a stage like 

 the eyes of Calocaris, which are practically sessile, have lost all of the normal visual elements, 

 and have only colorless pigment, but still present a large, flattened, transparent cornea at the 

 anterior margin of the carapace. 



"It is interesting to note that the highly modified eyes of Pentacheles are found in a well- 

 defined group, all the species of which have probably been inhabitants of deep water for 

 considerable geological periods; while the equally deep-water species, with less modified or 

 obsolescent eyes, are much more closely allied to shallow-water species, from whose ancestors 

 they may have been derived in comparatively recent times." * 



It seems from the foregoing statements that a large proportion — over one-third — of these 

 deep-sea Crustacea have eyes in a greater or less degree of degeneration from disuse, and that the 

 greater proportion are inhabitants of the deepest and consequently darkest portion of the ocean 

 depths. It is possible that future researches may show that the forms with well-developed eyes 

 are twilight forms, which live between the dimly lighted superficial and the deepest layers of the 

 water, and not wholly restricted to the totally dark abysses. Moreover, it may be found that the 

 forms without eyes burrow in the ooze or live under loose objects at the bottom, and thus live in 

 a darkness still more profound than those which simply hover over the bottom. At all events 

 there is, as different writers have observed, a striking parallelism between the deep-sea blind 

 Crustacea and those inhabiting caves, and this seems due to a single cause — the absence of light. 

 It will be a matter of great interest to make careful researches on the finer structure of the rudi- 

 mentary eyes, and on the alterations which may have taken place in the brain, particularly the 

 optic lobes and nerves, in order to ascertain whether they have been modified as in cave animals. 

 As observed by A. Milne-Edwards and others, these blind or eyeless deep-sea Crustacea also 

 resemble the cave forms in often having slender, elongated bodies and very much attenuated 

 antennae and limbs to compensate for the loss of eye-sight. 



Professor Smith also states that the "large size and small number of the eggs is a very marked 

 characteristic of many deep sea Decapoda." This is also true of the cave eyeless spider Anthrobia 

 mammouthia. whose eggs, as we have previously remarked, are proportionately very large and few 

 in number (PI. XX, fig. 16). As to the eggs of other cave animals, our knowledge is still too 

 imperfect to allow us to institute auy further comparison. 



Annala aud Mag. Nat. Hist., March, 188C, pp. 194-197. 



