200
J. A. 0. NIOOL
fish) apparently can do this whereas sunfish (from the northern hemisphere) cannot (Hasler and Schwassmann, 1960).
Salmon, tuna and eels perform long sea migrations which occur at a
relatively slow late and during which they may be able to adjust
gradually to changing photoperiod and sun-arc. Moreover, salmon in
pelagic waters are known to carry out some of their migrations at night,,
when sun-compass reactions are not possible ; other guiding factma
must then be operative. On approaching parent streams or spawning
grounds, other sensory cues must come into play (Hasler, 1960a, b).
X. SYNOPSIS
Light-sensitive areas of cyclostomes are the skin, spinal cord, brain,
pineal complex, and paired eyes when present. The hag is practically
eyeless and has integumentary photoreceptors at the anterior end of
the head and in the cloaca1 region. The caudal region of the ammometea
is especially sensitive to light ; photoreceptors additional to the pineal
complex and the paired eyes occur in the head of the lamprey.
Integumentary photoreceptors occur in blind cave fishes and may
be present in some marine fishes. The pineal region of many teleosta is
sensitive to light and mediates some behavioural and colour responses.
Chromatophores over the pineal region control the amount of light
reaching the pineal complex. Pupillary movement takes place in aome
benthic selachians and teleosts. An occlusible tapetum lucidum liea in
the chorioidea of pelagic elasmobranchs and movement of pigment
across the tapetum, in light or darkness, takes place slowly. Retinomotor changes-radial excursions of receptors and retinal pigment-in
teleosts also occur slowly, in 30 to 60 min, and are related to change8
in light-intensity. Threshold value8 of mxwd epecies have been determined.
A slight degree of hypermetropia is postulated from retinoscopy of
selachian and teleost eyes, the significance of which awaits explanation.
The aplanatic condition and very short focal length of the teleost lens
are thought to be the result of its having a changing refractive index
from the centre to the periphery. When accommodating, the lens ia
displaced backwards along the antero-posterior axis and brings distent
objects into focus on the posterior retina,
Rhodopsins, based on vitamin A, aldehyde, are characteristic of
marine fishes. They have a great range of absorption maxima, ranging
from maxima around 480 mp (visual golds or chrysopsins of deep-sea
species) to purplo rhodopsins with maxima around 610 mp in fishes of
turbid coastal waters. The absorption characteristics of theee various
scotopic visual pigments seem to be related to the quality of light in the
J. A. 0. NIOOL
fish) apparently can do this whereas sunfish (from the northern hemisphere) cannot (Hasler and Schwassmann, 1960).
Salmon, tuna and eels perform long sea migrations which occur at a
relatively slow late and during which they may be able to adjust
gradually to changing photoperiod and sun-arc. Moreover, salmon in
pelagic waters are known to carry out some of their migrations at night,,
when sun-compass reactions are not possible ; other guiding factma
must then be operative. On approaching parent streams or spawning
grounds, other sensory cues must come into play (Hasler, 1960a, b).
X. SYNOPSIS
Light-sensitive areas of cyclostomes are the skin, spinal cord, brain,
pineal complex, and paired eyes when present. The hag is practically
eyeless and has integumentary photoreceptors at the anterior end of
the head and in the cloaca1 region. The caudal region of the ammometea
is especially sensitive to light ; photoreceptors additional to the pineal
complex and the paired eyes occur in the head of the lamprey.
Integumentary photoreceptors occur in blind cave fishes and may
be present in some marine fishes. The pineal region of many teleosta is
sensitive to light and mediates some behavioural and colour responses.
Chromatophores over the pineal region control the amount of light
reaching the pineal complex. Pupillary movement takes place in aome
benthic selachians and teleosts. An occlusible tapetum lucidum liea in
the chorioidea of pelagic elasmobranchs and movement of pigment
across the tapetum, in light or darkness, takes place slowly. Retinomotor changes-radial excursions of receptors and retinal pigment-in
teleosts also occur slowly, in 30 to 60 min, and are related to change8
in light-intensity. Threshold value8 of mxwd epecies have been determined.
A slight degree of hypermetropia is postulated from retinoscopy of
selachian and teleost eyes, the significance of which awaits explanation.
The aplanatic condition and very short focal length of the teleost lens
are thought to be the result of its having a changing refractive index
from the centre to the periphery. When accommodating, the lens ia
displaced backwards along the antero-posterior axis and brings distent
objects into focus on the posterior retina,
Rhodopsins, based on vitamin A, aldehyde, are characteristic of
marine fishes. They have a great range of absorption maxima, ranging
from maxima around 480 mp (visual golds or chrysopsins of deep-sea
species) to purplo rhodopsins with maxima around 610 mp in fishes of
turbid coastal waters. The absorption characteristics of theee various
scotopic visual pigments seem to be related to the quality of light in the