132 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



As regards the species of Cymothoa referred to by Semper, and the few Cyclopidse 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 Polydesmidse 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-cavernieolous 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 Cha^togaster, 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 Cephalopbora 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 Chitonidse, 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" (Glaus) is difficult to explain, 

 unless part of their life is spent at great depths in the sea, below the lighted portions.* 



Besides Machaerites and Hadenoecus, 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 cavernarum, 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 Sacken'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 Polydesmidae are blind. 



It has been objected f 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 G. 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 antennae and palpi, are due solely to 

 the influence of light upon the development of the larvae. 



* Tentacular eyes occur in the following genera 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. Dall's letter on p. — . 



t F. W. Putnam, Amer. Naturalist, January, 1872. 



