FISHES 



305 



(with the exception of the eel) either by the falciform process or a 



hyaloid system of vessels. The pigmentary eijithelium has a normal 



configuration (Fig. 343), but in some species (Cyprinidse, Percidae) has an 



occlusible retinal tapetum lucidum of varying extent, sometimes 



small, sometimes occupying a large oval area or almost the entire 



fundus. In the region thus occupied the epithelial cells have long 



processes heavily packed with crystals of guanine or 



a guanine-like compound containing calcium ; in 



dim light the fuscin pigment migrates backwards 



into the cell-bodies exposing a silvery mirror of 



guanine ; in bright light the dark pigment migrates 



through the guanine layer to the tips of the processes, 



covering up the tapetum and absorbing the excess of 



incident light (SchiefFerdecker, 1887 ; AbelsdorfF, 



1896; Garten, 1907; Wunder, 1925-30). Occasionally 



in abyssal fishes which are never exposed to bright 



light {Evermanella), the pigment does not migrate 



and is confined to small masses at the ends of the cell 



processes, an arrangement also seen in Chondro- 



steans. 



The visual retina has received much study (Figs. 

 344-(5).^ This structure in Teleosteans is remarkable 

 among Fishes for the regularity of its layers and the 

 absence of displaced elements, the thickness of the 

 nuclear layers and the number of ganglion cells ; it 

 is the most highly differentiated retina among the 

 Fishes and compares in this respect only with the 

 highest Vertebrates. Typically both rods and cones 

 are found : only rarely as in deep-sea species {Batliy- 

 troctes) and exceptionally in fresh- water types 

 (Hiodon) are the cones absent (Moore, 1944). In 

 deep-sea forms, in order to increase the sensitivity to 

 light, the rod population is usually dense and may 

 indeed be the liighest among all Vertebrates (5,000,000 sq. mm. in 

 Lampayiyctus — Vilter, 1951) (Wunder, 1925-30) while the individual 

 elements may be elongated ; in a bathypelagic species, Bathylagus 

 benedicti, they are arranged in three distinct rows (Vilter, 1953). 



C^C 



Fifj. 343. — The 

 Pigment Epi- 

 thelium OF THE 

 Goldfish, 

 C A /! A s s / u s 

 A U It A T u .y. 



Ill the light- 

 adapted state. 

 The processes con- 

 tain migratory pig- 

 ment in rod -like 

 granules concentra- 

 ted mainly in their 

 tips, r and c repre- 

 sent spaces occu- 

 pied by rods and 

 cones (after Walls). 



Presumably as an adaptation to increase the visual acuity in the direction 

 in which food is usually obtained, different areas of the retina frequently vary 

 in the relative density of the population of rods and cones ; thvis in the ininnow, 

 Ericymba, which frecjuents the bottom, the ventral area of the retina contains 



1 H. Muller (1857), M. Schultze (1866), Dobrowolskv (1871), W. Muller (187.5), 

 Hannover (1876), Denissenko (1881), W. Krau.se (1886), Cajal (1893-1933), Greeff (1899), 

 Hesse (1904), Wunder (1925-30), Arev (1928), Verrier (1928-38), Mayou (1933), 

 Rochon-Duvigneaud (1943), Vilter (1947-54), Sverdliek (1954), H. Muller (1954). 



S.O. — VOL. I. 20 



