FISHES 



329 



they remain constantly at the bottom of the sea, they he upon one 

 side so that the eye wliich finds itself underneath (the left eye in the 

 sole, the right in the turbot) migrates to the upper side and eventually 

 lies alongside the other in a hole formed in the frontal bone. The two 

 orbits, like the rest of the head, are consequently very asymmetrical. 

 In one species (Psetfodes) the migration is incomplete so that the 

 migratory eye does not reach the top of 

 the head. In this way the flat-fish attams 

 a wide binocular field above^, and in many 

 species the eyes are raised on ocular 

 turrets so that vision is still possible when 

 most of the body of the fish is concealed 

 under sand. In order to avoid dazzle in 

 the uj) ward-looking eyes of these flat- 

 fishes, as well as in some other bottom 

 fishes, an expansile pupillary operculum is 



developed comparable to that found in Batoidei.- Tliis structure may 

 be small, as in the star-gazer, Uranoscoi^us (Fig. 3'.»3) or so large that 

 it practically occludes the entire juipil. as in the cat-fish, Plecosfomus 

 (Fig. 394). ^ 



Fig. 393. 



Fig. 394. 



Fig. 393. — The Pupil of the 



StAKGAZEB, U RAyOSCOPVS. 



Fig. 394. — The Pupil of the 

 Catfish, Plecostomus. 



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(1931). 

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1 p. 679. 



Bruesrh and Arev- J- comp. XcuroL, 77, 



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■- p. 287. 



Psettodes 



