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THE EYE IN EVOLUTION 



noted, however, they may remain large and relatively well formed if 

 development from the larval to the adult stage is artificially conducted in 

 bright illumination (Kammerer, 1912) (Figs. 870-1). In natural conditions, 

 however, they are concealed under the skin, microscopic and either capable 

 merely of a directional light sense, as in limicoline types, or functionless, as 

 in cave-living types. Other Urodeles, on the other hand, such as the 

 newt (Triturus) or the North American axolotl, Amhy stoma, have rela- 

 tively simple but effective eyes, lacking iris folds and with a spherical lens, 



while in terrestrial salamanders the eyes, though 



small, are well formed.^ 



Among the cave forms the olm, Proteus may be 

 taken as a typical example, several species of which 

 inhabit the caves of Carinthia and Dalmatia (Fig. 869.) 

 The eyes of the adult Proteus anguinus are minute 

 spheres less than 0-5 mm. in diameter seen as shadows 

 deep underneath the skin. They form simple vesicles 

 without cornea or lens ; originally a inere accumulation 

 of epidermal cells within a capsule, the capsule disinte- 

 grates and the cells of the lens are replaced by connective 

 tissue (Fig. 870). The ocular cavity is almost entirely 

 taken up by a retina of a most rudimentary type and 

 between it and the external epithelium lie the open 

 remains of the optic vesicle. The visual cells are globular 

 and bear no resemblance to rods or cones ; there is an 

 elementary nuclear layer and a reticular layer while the 

 optic nerve is vestigial and largely neuroglial (Fig. 872) 

 (Configliachi and Rusconi, 1819 ; Desfosses, 1882 ; von 

 Hess, 1889 ; Kohl, 1889-92 ; Benedetti, 1922 ; 

 Stadtmiiller, 1929). 



Fig. 



872. — The Retina of 

 Proteus axguinus. 



visual elements (after Kohl). 



Similarly rudimentary eyes, even more degenerate than those of their 

 cavernicolous European relatives, are found in related types such as the 

 American blind salamanders, TyjMomolge, inhabiting the underground 

 streams of Texas, and Haideotriton ; such eyes are functionless. The eyes 

 of Typhlotriton, however, a salamander found in the caves of the Mississippi 

 Valley, normal in the larva but degenerate in the adult, are more fully formed 

 with a lens and a considerable degree of retinal differentiation although the 

 rods and cones disappear in the fully grown animal ; these constitute a link 

 between the degenerate eyes of the Proteidae and the normal urodelan eye 

 (Eigenmann, 1909). 



THE FOSSORIAL HABIT 



The FOSSORIAL OR BURROWING HABIT has led to the regression of the 

 eyes of many types of Invertebrates and Vertebrates. 



Thus among worms which burrow on the land {Lmnbricus terrestris) we 

 have already seen that the visual organs are of a very primitive type,- while 



p. 346. 



190. 



