724 THE EYE IN EVOLUTION 



absent ; in this event it is interesting that some species maintain projicience 

 by developing long filamentous " feelers " (the " feeler fish," Bathypterois). 

 It is true that most of the inhabitants of the sea-bottom retain their eyes and 

 that in some families these are neither unusually large nor small (such as the 

 grenadiers, Coryphsenoididse) ; it is also true that the only biological value 

 of these visual organs is to catch the fitful gleams of luminescence ; but it is 

 also true that many lose them (Alcock, 1902). 



Thus among Selachians the eyes are vestigial in several families of the rays — 

 TyiMonarke, Bengalichthys and Benthohatis . The eye of the last, for example, has a 

 crude cornea, a rudimentary iris, an undifferentiated retina, and no lens (Fig. 866) 

 (Brauer, 1908). Among Teleosts in some deeply bathypelagic forms such as Saccopharynx 

 and Cetomimus the eye is vestigial. In the latter the oval globe is only 0-7 inm. in 

 diameter, the lens and retina are rudimentary and the pigment epithelium unusually 

 thick (Brauer, 1908). Among some benthonic Teleosts the eyes may be still more 

 rudimentary and covered with opaque skin — Barathronus, Typhlonus, Aphyomis, and 



Fig. 867. — The Blind Deep-sea Teleost, Ipxops aoassizi. 

 Found at 2,000 m. (^ natural size) (after Garman, Albatross Report, 1899). 



Tauredophidiwm. An inhabitant of the ocean floor, Ipnops,^ is the only Vertebrate 

 known to have no trace of eyes (Eigenmann, 1909) ; this is a small black fish with 

 two luminous areas (resembling lanterns) in its head under the translucent bones of 

 the skull where the eyes might be expected, possibly adaptations of these organs 

 (Fig. 867). 



THE CAVEBNICOLOUS OR LIMICOLINE HABIT 



A CAVERNicoLOUS OR LIMICOLINE HABIT, whereby life is spent in the 

 darkness of caves or crevices or in a similarly lightless environment in mud or 

 beneath stones, also leads to a tendency for ocular regression. This is seen 

 among cave-dwelling worms such as the planarian Kenkiidse, or among 

 Arthropods inhabiting a similar environment. In the latter phylum typical 

 examples are seen in two species of Onychophores, Peripatopsis alba which 

 lives in lightless caves, and Typhloperipatus, found under rocks; in the 

 cavernicolous beetle, Anophthalmus, which is possessed only of a dermal light 

 sense (Marchal, 1910) ; the eyeless white cave-crayfish, Cambarus ayersii, 

 which retains some light-sensitivity in its cerebral ganglion (Wells, 1952) ; 

 and the cave-spiders {Anthrobia) which are entirely sightless ; but the 

 phenomenon is most markedly seen in cave-dwelling Fishes and Amphibians. 



^ iTTvos, a lantern ; wip, eye. 



