8
F. W. MUNL
better adapted for close-up vision than open-water fishes.” Some of the
species examined in the earlier study, however, are benthic (e.g.,
Prionotus, Paralichthys, and Pseudopleuronectes ) and would presumably
gain advantage from emmetropic rather than hypermetropic sight. An
abstract of recent work by Bogatyrev (1966) indicates that fishes can
focus their eyes for sharp vision over a wide range, from a near point of
5 cm or less out to infinity, in apparent agreement with the conclusions
of Pumphrey. This problem merits further study.
In addition to accommodation, several other devices can have somewhat the same result, but without requiring any active mechanism
(Walls, 1942).
(1) A pinhole pupil produces a fairly sharp image regardless of the
distances from it to the object and to the retina. When light adapted,
such elasmobranchs as Scyliorhinm and Raja have a pupil with a very
small aperture.
( 2 ) A “ramp” retina, which is tilted away from the lens, could simultaneously have in focus images of objects located at different distances.
In Raja, the upper portion of the retina is farther from the lens than the
lower portion. Objects nearby on the ocean bottom could, therefore, be
in focus at the same time as distant objects located above the animal.
( 3 ) The fact that the outer segments of the visual cells have considerable length means that objects at various distances would be equally in
(or out of! ) focus; presumably this has more to do with increasing sensitivity than with accommodation.
(4) Another structural modification is to have the eye permanently
set for vision at two particularly useful distances; Walls suggested the
analogy of bifocal spectacles. Subdivision of the retina into two parts in
the tubular eyes of certain deep-sea fishes is one example (Section I, D).
Better known is the “four-eyed” fish ( Anubbps; see Walls, 1942; Schwassman and Kruger, 1965). Anableps swims at the surface with its eyes
partly out of water. The pupil of the light-adapted eye is divided horizontally by flaps of the iris. Objects in air are imaged on the ventral part
of the retina, those in water on the dorsal part. The lens is egg-shaped;
compensating for the lack of corneal refraction under water, there is
greater curvature at the lens surfaces concerned with aquatic than with
aerial vision. The two parts of the retina are specialized for their different
functions ( Inouye and Noto, 1962; Schwassman and Kruger, 1965).
C. Light and Dark Adaptation
Gnathostome fishes can change the effective light intensity at the
receptor level by several means. Pupil movement and photomechanical
F. W. MUNL
better adapted for close-up vision than open-water fishes.” Some of the
species examined in the earlier study, however, are benthic (e.g.,
Prionotus, Paralichthys, and Pseudopleuronectes ) and would presumably
gain advantage from emmetropic rather than hypermetropic sight. An
abstract of recent work by Bogatyrev (1966) indicates that fishes can
focus their eyes for sharp vision over a wide range, from a near point of
5 cm or less out to infinity, in apparent agreement with the conclusions
of Pumphrey. This problem merits further study.
In addition to accommodation, several other devices can have somewhat the same result, but without requiring any active mechanism
(Walls, 1942).
(1) A pinhole pupil produces a fairly sharp image regardless of the
distances from it to the object and to the retina. When light adapted,
such elasmobranchs as Scyliorhinm and Raja have a pupil with a very
small aperture.
( 2 ) A “ramp” retina, which is tilted away from the lens, could simultaneously have in focus images of objects located at different distances.
In Raja, the upper portion of the retina is farther from the lens than the
lower portion. Objects nearby on the ocean bottom could, therefore, be
in focus at the same time as distant objects located above the animal.
( 3 ) The fact that the outer segments of the visual cells have considerable length means that objects at various distances would be equally in
(or out of! ) focus; presumably this has more to do with increasing sensitivity than with accommodation.
(4) Another structural modification is to have the eye permanently
set for vision at two particularly useful distances; Walls suggested the
analogy of bifocal spectacles. Subdivision of the retina into two parts in
the tubular eyes of certain deep-sea fishes is one example (Section I, D).
Better known is the “four-eyed” fish ( Anubbps; see Walls, 1942; Schwassman and Kruger, 1965). Anableps swims at the surface with its eyes
partly out of water. The pupil of the light-adapted eye is divided horizontally by flaps of the iris. Objects in air are imaged on the ventral part
of the retina, those in water on the dorsal part. The lens is egg-shaped;
compensating for the lack of corneal refraction under water, there is
greater curvature at the lens surfaces concerned with aquatic than with
aerial vision. The two parts of the retina are specialized for their different
functions ( Inouye and Noto, 1962; Schwassman and Kruger, 1965).
C. Light and Dark Adaptation
Gnathostome fishes can change the effective light intensity at the
receptor level by several means. Pupil movement and photomechanical