716 THE EYE IN EVOLUTION 



has an independent parietal nerve associated with a near by parietal centre, 

 a connection which in many cases is transitory and degenerates before 

 maturity so that the organ would appear to lose its function. In these 

 species the pineal body is always rudimentary and the vesicle is usually 

 absent. It is also interesting that the presence of a parietal opening in the 

 roof of the skull of fossil labyrinthodont amphibians and extinct reptiles of 

 the Palaeozoic and Mesozoic eras suggests that a functional eye existed 

 in these species also. In the more recent reptiles, such as geckos, snakes, 

 tortoises, turtles, crocodiles, and alligators, the eye-structure disappears and 

 the epiphyseal arch gives rise to a glandular organ, an arrangement retained 

 in the higher animals. In some birds and mammals analogous rudiments 

 appear in embryonic life which disappear with 

 development,^ but in these types the pineal organ 

 has a glandular structure and lies snugly hidden on 

 the roof of the diencephalon between the cerebrum 

 and the cerebellum (Fig. 861). It is thus evident 

 that the pineal organ constitutes a definitive eye only 

 in the lamprey and to a less extent in certain primi- 

 tive "ganoid " fishes, while the parietal organ forms an 

 eye-like structure in the lamprey and also in primitive 

 Fig. 862. — The Lam- reptiles ; Otherwise the latter organ is vestigial. 



PREY, PetROMYZOS. 



Dorsal View of the head ^j^^ median eye of the lamprey lies under a 



end oi the animal show- , ,• , r- , , ^ ■ ,-, ■ ^^■ c ,^ 



ine the eve E the nasal localized area oi transparent skin on the inidline ot the 



aperture, N , and the dorsal surface of the head immediately behind the single 

 pineal area, Pin. median nostril (Fig. 862). It consists of two diverticula lying 



vertically one upon the other (Ahlborn, 1883 ; Beard, 1889 ; 

 Stiidnicka, 1905 ; Dendy, 1907 ; Mygind, 1949). The more superficial and dorsal 

 vesicle is the jDineal, the lower the parietal eye (Fig. 856). Together they form an organ 

 incapable of optical iinagery but doubtless able to appreciate differences in light in- 

 tensity. Of the two the pineal eye is the more elaborately developed (Figs. 863 and 864). 

 It forins a vesicle lying directly underneath the skin ; the cells of the superficial wall 

 are elongated to form a flat and imperfect lens ; those of the deeper wall form a 

 pigmented retina comprised of sensory and supporting cells, ganglion cells and nerve 

 fibres which pass as the pineal nerve in the posterior coiTimissure to the right habenular 

 ganglion. The retinal pigment is of two types — a dark melanin-like pigment and 

 whitish -yellow granules corresponding closely to the guanine -like pigment of the skin ; 

 the first has an absorbent, the second probably a reflective function analogous to the 

 similar pigment in the compound eyes of some Arthropods. The free ends of the 

 sensory ceils face the lumen of the vesicle which is fllled with a nucleated syncytial 

 " vitreous." The parietal organ forms a somewhat sunilar vesicle of simpler construc- 

 tion, varying considerably in size ; the rudimentary parietal nerve leads through the 

 habenular commissure to the left habenular ganglion. 



THE MEDIAN EYE OF LiZABDS and Sphenodou 2 is derived from the parietal body 

 and forms a remarkably eye-like organ (Spencer, 1886 ; Leydig, 1887 ; Strahl and 

 Martin, 1888 ; Klinckowstrom, 1893 ; Vu-chow, 1901 ; Studnicka, 1905 ; Dendy, 



^ Pigeon (Livini, 1905), guinea-pig (Chiarugi, 1919), ox (Favaro, 1904). 

 2 p 379, 



