8 & 
MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 
species of Glyphocrangon 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 sulcati/rons, P. cristata , Acan- 
tliephyra microphthalmia , 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 Glyphocrangon, 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 function less; 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 Ponthophilus ahyssi 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 Peutacheles 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-Ed wards 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 any further comparison. 
Annals and Mag. Nat. Hist., March, 1886, pp. 194-197. 
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