178
J. A. 0. NICOL
the common dogfish (Sc$iorhinzcs canicula) aeas an approaching net and
is quite adept at avoiding capture (Bateson, 1890~). And recent work
hw shown that in clear water vision plays a predominant role in
guiding sharks to prey at distances less than 50 ft (Gilbert, 1962).
D. Retinomotor Changes in Tekosts
In many teleosts movements take place in the retina which am
collectively termed retinomotor movements. They involve dgration
of retinal pigment and displacement of rods and cones under the
influence of light and other factors. The retinal pigment is contained
in cells of the pigment epithelium, which bear processes that extend
inwards between the receptor cells. In darkness, the retinal pigment
moves outwards, towards the base of the pigment cell (withdrawd);
in light, the pigment moves inwards (expansion). When movement of
the rods and cones occur^, the rod myoids contract and the oone
myoids lengthen in darkness ; in light, the rod myoids lengthen and the
cone myoids contract. Considered in conjunction, these several movements cause the outer segments of the rods to be exposed in darknese ;
in light, the outer segments of the con- are withdrawn towards the
interior of the eye, whereas those of the rods are obscured. These
changes, seemingly, are devoid of visual significance in total darknepls ;
they are concerned with enhancing vision in bright and dim light.
Retinomotor changes are to be found in fishes inhabiting inahore
waters and surface waters of the oceans. They do not occur in bathypelagic and bathybenthic species. In eels (Anguilla anguilla) the rode
migrate whereaa the cones do not; the eye of the conger (Conger
conger) contains few or no cones, and it is uncertain whether the rode
move (Garten, 1907). It is believed that only the pigment m o m in
flat-fishes (pleuronectids), and that rods and cones are stationary
(Bayliss et al., 1936). It has since been discovered that cones of flat-fish
extend and retract in darkness and in light, and a report concerning
soles ( S o h solea) has been published (Nicol, 1964b).
The movement of pigment, cones and rods takes place rather slowly
compared, for example, with the swift course of pupillary movement.
Timea given in the literature for maximal contraction or expamion
range from 20 to 70 min for various species and conditiona of observation. I n Ameiurus and Fundulus the retinal pigment attaine full
expansion or retraction in 1 hr ; cones shorten and rods lengthen (in
light) in 45 min ; reverse changes in cones and rods occupy 30 min or
more. I n Abrumis the pigment requires 45 min to expand and 30 min
to retract ; cones shorten in 45 min and extend in 30 min (Arey, 1916).
In Leuciscus pigment expands and retracts in 20 min to I hr; cones
J. A. 0. NICOL
the common dogfish (Sc$iorhinzcs canicula) aeas an approaching net and
is quite adept at avoiding capture (Bateson, 1890~). And recent work
hw shown that in clear water vision plays a predominant role in
guiding sharks to prey at distances less than 50 ft (Gilbert, 1962).
D. Retinomotor Changes in Tekosts
In many teleosts movements take place in the retina which am
collectively termed retinomotor movements. They involve dgration
of retinal pigment and displacement of rods and cones under the
influence of light and other factors. The retinal pigment is contained
in cells of the pigment epithelium, which bear processes that extend
inwards between the receptor cells. In darkness, the retinal pigment
moves outwards, towards the base of the pigment cell (withdrawd);
in light, the pigment moves inwards (expansion). When movement of
the rods and cones occur^, the rod myoids contract and the oone
myoids lengthen in darkness ; in light, the rod myoids lengthen and the
cone myoids contract. Considered in conjunction, these several movements cause the outer segments of the rods to be exposed in darknese ;
in light, the outer segments of the con- are withdrawn towards the
interior of the eye, whereas those of the rods are obscured. These
changes, seemingly, are devoid of visual significance in total darknepls ;
they are concerned with enhancing vision in bright and dim light.
Retinomotor changes are to be found in fishes inhabiting inahore
waters and surface waters of the oceans. They do not occur in bathypelagic and bathybenthic species. In eels (Anguilla anguilla) the rode
migrate whereaa the cones do not; the eye of the conger (Conger
conger) contains few or no cones, and it is uncertain whether the rode
move (Garten, 1907). It is believed that only the pigment m o m in
flat-fishes (pleuronectids), and that rods and cones are stationary
(Bayliss et al., 1936). It has since been discovered that cones of flat-fish
extend and retract in darkness and in light, and a report concerning
soles ( S o h solea) has been published (Nicol, 1964b).
The movement of pigment, cones and rods takes place rather slowly
compared, for example, with the swift course of pupillary movement.
Timea given in the literature for maximal contraction or expamion
range from 20 to 70 min for various species and conditiona of observation. I n Ameiurus and Fundulus the retinal pigment attaine full
expansion or retraction in 1 hr ; cones shorten and rods lengthen (in
light) in 45 min ; reverse changes in cones and rods occupy 30 min or
more. I n Abrumis the pigment requires 45 min to expand and 30 min
to retract ; cones shorten in 45 min and extend in 30 min (Arey, 1916).
In Leuciscus pigment expands and retracts in 20 min to I hr; cones