4
F. W. MUNZ
Outermost, adjacent to the choroid, is the pigment epithelium (Section
I, C). The visual cells of lampreys are usually of two morphologically distinct types, but their affinities with rods and cones of other vertebrates
are not certain (Walls, 1942). Elasmobranchs and teleosts each depart
from the familiar duplex pattern of vertebrates, but in different ways.
Most elasmobranchs are thought to have pure-rod retinae, but some
sharks ( Mustelus, Lamna, Squutina, Negaprwn, Carcharinus, Sphyrna,
and Ginglymostm) and rays (Myliobatis and Dasyatis) are reported
to have cones as well as rods (Walls, 1942; Rochon-Duvigneaud, 1943;
Gruber et al., 1963; Hamasaki and Gruber, 1965; Tamura and Niwa,
1967). In contrast, teleosts typically have both cones and rods. In addition
to the ordinary, single cones, they also have peculiar visual cells called
“twin cones.” These differ from the double cones found in most vertebrate groups, in that the “twins” in each cell pair are morphologically
similar and fused longitudinally. Certain teleosts, in which a tapetum
is well developed, lack single cones (e.g., anchovies; see Tamura, 1957;
OConnell, 1963). In many deep-sea fishes, cones are entirely absent
(Section I, D).
It is probably typical of teleosts that the visual cells are not distributed
uniformly over the retina. Commonly there is a specialized temporal
“area” in which cones are more numerous. The area may represent a
concentration of twin cones alone (as in anchovies, which lack single
cones) or of both twin and single cones ( OConnell, 1963). A cone-rich
area also occurs in the shark, Mustelus. The fovea is a further retinal
specialization for increased visual acuity and consists of a depression or
pit in the retina, overlying the area of more numerous (and often smaller)
visual cells. Blood vessels are typically absent from the fovea. The occurrence of a fovea is accompanied by the ability to fixate the image of a
moving object on this special region by voluntary eye movements. Fixation is normally associated with binocular vision. In the laterally placed
eyes of fishes, it is not surprising, therefore, that foveae are usually
located near the posterior (temporal) border of the retina. Rods and
twin cones are nearly or entirely absent in the fovea, which has concentrations of single cones. The pupil is usually larger than the lens in these
eyes; in other words, there is an aphakic space anterior to the lens (Walls,
1942). This reduction of the iris allows light from objects directly ahead
of the fish to be focused on the fovea. Retinal images of objects viewed
to the side must be degraded, of course, by light passing through the
aphakic space. Actually, foveae are rare in teleosts; they have been described in about 20 littoral marine species (Kahmann, 1936; Baron and
Verrier, 1951). Foveae have also been reported in freshwater species of
Fundulus and Umbridw (Prince, 1956). Temporal foveae occur in the
F. W. MUNZ
Outermost, adjacent to the choroid, is the pigment epithelium (Section
I, C). The visual cells of lampreys are usually of two morphologically distinct types, but their affinities with rods and cones of other vertebrates
are not certain (Walls, 1942). Elasmobranchs and teleosts each depart
from the familiar duplex pattern of vertebrates, but in different ways.
Most elasmobranchs are thought to have pure-rod retinae, but some
sharks ( Mustelus, Lamna, Squutina, Negaprwn, Carcharinus, Sphyrna,
and Ginglymostm) and rays (Myliobatis and Dasyatis) are reported
to have cones as well as rods (Walls, 1942; Rochon-Duvigneaud, 1943;
Gruber et al., 1963; Hamasaki and Gruber, 1965; Tamura and Niwa,
1967). In contrast, teleosts typically have both cones and rods. In addition
to the ordinary, single cones, they also have peculiar visual cells called
“twin cones.” These differ from the double cones found in most vertebrate groups, in that the “twins” in each cell pair are morphologically
similar and fused longitudinally. Certain teleosts, in which a tapetum
is well developed, lack single cones (e.g., anchovies; see Tamura, 1957;
OConnell, 1963). In many deep-sea fishes, cones are entirely absent
(Section I, D).
It is probably typical of teleosts that the visual cells are not distributed
uniformly over the retina. Commonly there is a specialized temporal
“area” in which cones are more numerous. The area may represent a
concentration of twin cones alone (as in anchovies, which lack single
cones) or of both twin and single cones ( OConnell, 1963). A cone-rich
area also occurs in the shark, Mustelus. The fovea is a further retinal
specialization for increased visual acuity and consists of a depression or
pit in the retina, overlying the area of more numerous (and often smaller)
visual cells. Blood vessels are typically absent from the fovea. The occurrence of a fovea is accompanied by the ability to fixate the image of a
moving object on this special region by voluntary eye movements. Fixation is normally associated with binocular vision. In the laterally placed
eyes of fishes, it is not surprising, therefore, that foveae are usually
located near the posterior (temporal) border of the retina. Rods and
twin cones are nearly or entirely absent in the fovea, which has concentrations of single cones. The pupil is usually larger than the lens in these
eyes; in other words, there is an aphakic space anterior to the lens (Walls,
1942). This reduction of the iris allows light from objects directly ahead
of the fish to be focused on the fovea. Retinal images of objects viewed
to the side must be degraded, of course, by light passing through the
aphakic space. Actually, foveae are rare in teleosts; they have been described in about 20 littoral marine species (Kahmann, 1936; Baron and
Verrier, 1951). Foveae have also been reported in freshwater species of
Fundulus and Umbridw (Prince, 1956). Temporal foveae occur in the