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MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
As regards the species of Cymothoa referred to by Semper, and the few Cyclopidte which are 
eyeless, we need to know more concerning their habits, whether they do not for the most part live 
in dark places, as do the eyeless species of the Myriopodous families Polydesmidic and Polyzomidse, 
which have apparently little use for organs of vision. Indeed, it may be laid down as an axiomatic 
truth that where eyes are defective or atrophied, it is owing to disuse induced by physical sur¬ 
roundings of such a nature as to enable the animals in question to dispense with organs of sight. 
In the foregoing list of eyeless non-cavernicolous animals, other than those living in the abysses 
of the ocean or of lakes, it will be seen how very exceptional is the absence of eyes in orders and 
classes of animals in which they are generally present. In regard to Chsetogaster, the species of 
this genus are parasitic in their habits, and it is not improbable that most of the oligochete worms 
which are eyeless burrow in the mud, spending the greater part of their lives in dark places, where 
eyes would be of little or no service to them. 
The bivalve mollusks are generally eyeless from the same cause; the clam and probably 
nearly all the burrowing Lamellibranch mollusks, which live deep in the sand or mud, in partial 
or perhaps total darkness, can afford to dispense with the sense of sight; while the Pectens, Lima, 
Spondylus, Tellina, Pectunculus, Area, etc., which leave the mud or the sand to skip over the 
surface, have highly developed eyes, as do most of the Cephalophora and all Cephalopoda. The 
Scaphopoda, which live buried in the mud and are headless, have no eyes. Where the eyes of 
certain Cephalophora are absent, as in the Chitonidm, we have also a degeneration involving 
the loss of a distinct head and tentacles in adaptation to their sedentary mode of life. Why in 
Pteropods the eyes should be either “absent or very rudimentary” (Clans) is difficult to explain, 
unless part of their life is spent at great depths in the sea, below the lighted portions* * 
Besides Machserites and Hadencecus, Semper mentions Anthomyia, Phora, and species of 
Nesticus and Linyphia, which have “well developed” eyes but live in caves, also “Spirostrephon.” 
The latter-named Myriopod is undoubtedly Pseudotremia eavernarum, which, though it possess a 
facetted cornea, is, as we have shown, without an optic nerve, and is therefore blind. The Sco- 
terpes copei of Mammoth Cave, as we have also shown, has neither eyes nor optic lobes nor optic 
nerves. The spiders referred to occur near the opening of the smaller caves, and may be regarded 
as perhaps twilight forms. At any rate, the eyeless Anthrobia of Mammoth Cave usually has no 
other spiders associated with it. Until the brain and optic nerves of these forms have been 
investigated by carefully made microscopic sections, one can not tell whether the optic nerves are 
present or absent. Experience has taught us that the simple presence of pigmented eyes does 
not necessarily prove that the creature has the power of sight. 
The Phora fly, which passes its transformations in bats’ dung, is a twilight species, while the 
so-called Anthomyia is Osten Sackeu’s Blepliaroptera defessa, which is also a twilight species, 
being found near the mouth of the larger caves and in numerous smaller caves in different parts 
of the country from Virginia to Utah. It is possible that it may yet be found outside of caves. 
As regards the little cave in Utah which we examined, it is of slight extent, only dark at the 
end, and the insects found in it were all to be compared with the twilight fauna of such a cave as 
Mammoth or Wyandotte; beside that, all the species of Polydesmidse are blind. 
It has been objected! that darkness has had little or nothing to do with the atrophy of the 
eyes of the blind-fish, because in Chologaster of Mammoth Cave and subterranean waters the 
eyes are normal, while it was alleged that the tactile organs were wanting. Prof. S. A. Forbes 
has ably disposed of this objection in the American Naturalist for January, 1882. 
In a later number (March, 1881) of the same journal he briefly described a new species of 
Chologaster under the name C. papilliferus, taken from a spring in Union county, southern Illinois; 
upon the degree of light to which the specimens are exposed, maintains that the individuals with developed eyes are 
males and those with the eyes very small or entirely deficient are females. Grenier, in reply (1. c., p. 650), maintains 
that the characters upon which De Saulcy has founded his genus Linderia and those which he regards as indicative 
of sex, such as the development of the eyes and wings and the structure of the antennie and palpi, are due solely to 
the influence of light upon the development of the larv®. 
* Tentacular eyes occur in the following genei-a of Lamellibranchs: Pecten, Spondylus, Lima, Ostrea (?), Pinna, 
Pectunculus, Modiola, Mytilus (?), Cardium, Tellina, Mactra, Venus, Solen, Pholas, and Galeomma. (Lankester in 
Ency. Britt., art. Mollusca, p. 693). Compare also Mr. Dali’s letter on p.—. 
t F. W. Putnam, Amer. Naturalist, January, 1872. 
