FISHES 



collect as miicli as possible of the small amount of light available, and, 

 indeed, in some species in order to accommodate this structure the 

 eye may attain a size more than half the length of the head {Bathylagus, 

 Zenion, etc.) — relatively the largest eyes of all Vertebrates. A much 

 more economic arrangement may therefore be adopted by some deep- 

 sea Teleosts in the tubular eye (or telescojjic eye) wherein the 

 unnecessary volume of a relatively circular organ has been eliminated 

 in favour of a cylindrical shape, the axial portions only of the globe 



323 



Fig. 380. — The Tubular Eye of a Deep-sea Fish. 



The eye of Scopelarchus analis, an inhabitant of the deep Atlantic and 

 Indian oceans, in longitudinal section, showing the enormous lens and the 

 general distortion of the globe. C, the lens cushion moved by a muscle, .1/, 

 which accommodates for distance ; T, tapetum ; Ch, choroid ; i?j, accessory' 

 retina ; R^, princijjal retina ; O, optic nerve (after Chun). 



being retained in order that the enormous lens might be accommodated 

 in an organ which had not become imj)ossibly large (Fig. 380). In 

 such an e3^e the lens occupies the entire anterior portion of the globe 

 and the iris is eliminated. In order to increase the visual field, however, 

 the "principal retina "lying at the bottom of the tube may be reinforced 

 by an " accessory retina " continued up one side ojDposite which the 

 sclera remains transparent (Brauer, 1908). In these species the eye is 

 initially normal in form and becomes tubular as growth proceeds (the 

 hatchet fish, Argi/ropelecus, etc., Contino, 1939) (Figs. 376-7) ; in 

 some the eyes are eventually directed forwards ((r/f/f/M^j/rMs) (Fig. 379) or 



