194
J. A. C. NICOL
pelagic fishes (visual golds). Measurements made of the lights of lantern
fish show that the radiant flux ranges from 0.1 x 10-9 pJ (ca. 0-1 sec)
to 52 x
pW/cm, at 1 m distance (Nicol, 1960; Clarke et al., 1962).
In clear oceanic water it has been estimated that another fish would
see the light of a lantern fish at distances of 10 to 16 m. However, if a
fish had a scnsitivity 100 x that of man, it might dctcct luminescence
at 60 m or more. 1,uniincscotit flashctl are usually intermittent, and
this fciLture msy crihancc thrir ability to be detected (Nieol, 1958).
Absolute thrcsholds for retinomotor changes to tho " dark ' I condition in the eyes of various tdeoxts lie in the range 10 to 10 -3 pW/cm,
(p. 1x0). Since regulation of retinomotor responses has not been worked
out satisfactorily, it is uncertain what receptors arc involved.
Experiments on behaviour of the hagfish show that it responds by
general activity to illumination down to 6 m c (perhaps lower) ; this is
approximately equivalent t o 0.5 pW/cm2. Light levels of this order
occur in coastal waters of 30 m or so where Myzine is found (Gustafson,
1935; Newth and Ross, 1955). The light is detected by generalized
integumentary photoreceptors and a light level of 0.5 pWlcm2 may be a
representative value for absolute thresholds of such unspecialized
receptors.
VIII. THE CHORIOIDAL GLAND
The chorioidal gland found in the chorioidal layer of the eyes of
most teleosts is a horseshoe-shaped structure containing a rete mirabile.
Its blood supply comes from the pseudobranch via the ophthalmic or
efferent pseudobranchial artery, The arterial capillaries entering the rete
itltcrn:~t,c* rc~guli~rly with thr vc~~rotin wjillariiw 1wLviiig it,, WAII the vmoun
oiiLfl(~w pmi i t l l o t h ~ ~ t i ~ ~ r i ( ~ ~ ~ i ~ , ~ ~ i I I i i r i ~ !
'I'lioro in I I ~ O 11. I~v~t,tJt~rit~ ljo(iy 111
tIi(t vIiorioi(1 i i ( w 1 h ftiloiforiti iirovtw. It iH Hiipp1it-d by tho retinal
I I rl.c~y riritl i1.s v w o i i s oit14ow nlnt) gow to the ohorioixpillnris. A
( ~ o t i i i i ~ ~ t i
ftuhtro of both o1iorioidiLl gland and lentiform body is the
rcyrular dt~cmuLtk)ir of rtrterinl uiid venous capillaries within them,
arangenient that permits maximal exchange of material on the countercurrent principle (Barnett, 1951).
Parry and Holliday (1960) found that extirpation of the pseudobranch produced rapid darkening of the skin and, after some weeks,
degeneration of the chorioid gland (in Salmo, Clupea, etc.). They suggested that the pseudobranch produced a hormone capable of causing
expansion of ehromatophores, and that the chorioidal gland regulated
the entry of this substance into the general circulation. Retia arJRociated
with the swim-bladder are concerned with secretion of oxygen and it
has been found that the chorioidal rote seem8 actively to secrete oxygen
J. A. C. NICOL
pelagic fishes (visual golds). Measurements made of the lights of lantern
fish show that the radiant flux ranges from 0.1 x 10-9 pJ (ca. 0-1 sec)
to 52 x
pW/cm, at 1 m distance (Nicol, 1960; Clarke et al., 1962).
In clear oceanic water it has been estimated that another fish would
see the light of a lantern fish at distances of 10 to 16 m. However, if a
fish had a scnsitivity 100 x that of man, it might dctcct luminescence
at 60 m or more. 1,uniincscotit flashctl are usually intermittent, and
this fciLture msy crihancc thrir ability to be detected (Nieol, 1958).
Absolute thrcsholds for retinomotor changes to tho " dark ' I condition in the eyes of various tdeoxts lie in the range 10 to 10 -3 pW/cm,
(p. 1x0). Since regulation of retinomotor responses has not been worked
out satisfactorily, it is uncertain what receptors arc involved.
Experiments on behaviour of the hagfish show that it responds by
general activity to illumination down to 6 m c (perhaps lower) ; this is
approximately equivalent t o 0.5 pW/cm2. Light levels of this order
occur in coastal waters of 30 m or so where Myzine is found (Gustafson,
1935; Newth and Ross, 1955). The light is detected by generalized
integumentary photoreceptors and a light level of 0.5 pWlcm2 may be a
representative value for absolute thresholds of such unspecialized
receptors.
VIII. THE CHORIOIDAL GLAND
The chorioidal gland found in the chorioidal layer of the eyes of
most teleosts is a horseshoe-shaped structure containing a rete mirabile.
Its blood supply comes from the pseudobranch via the ophthalmic or
efferent pseudobranchial artery, The arterial capillaries entering the rete
itltcrn:~t,c* rc~guli~rly with thr vc~~rotin wjillariiw 1wLviiig it,, WAII the vmoun
oiiLfl(~w pmi i t l l o t h ~ ~ t i ~ ~ r i ( ~ ~ ~ i ~ , ~ ~ i I I i i r i ~ !
'I'lioro in I I ~ O 11. I~v~t,tJt~rit~ ljo(iy 111
tIi(t vIiorioi(1 i i ( w 1 h ftiloiforiti iirovtw. It iH Hiipp1it-d by tho retinal
I I rl.c~y riritl i1.s v w o i i s oit14ow nlnt) gow to the ohorioixpillnris. A
( ~ o t i i i i ~ ~ t i
ftuhtro of both o1iorioidiLl gland and lentiform body is the
rcyrular dt~cmuLtk)ir of rtrterinl uiid venous capillaries within them,
arangenient that permits maximal exchange of material on the countercurrent principle (Barnett, 1951).
Parry and Holliday (1960) found that extirpation of the pseudobranch produced rapid darkening of the skin and, after some weeks,
degeneration of the chorioid gland (in Salmo, Clupea, etc.). They suggested that the pseudobranch produced a hormone capable of causing
expansion of ehromatophores, and that the chorioidal gland regulated
the entry of this substance into the general circulation. Retia arJRociated
with the swim-bladder are concerned with secretion of oxygen and it
has been found that the chorioidal rote seem8 actively to secrete oxygen