172
J. A. C. NICOL
II. EXTRA-OCU- RECEPTION
Photoreception in many fishes may be mediated by structures other
than the eyes. Skin, spinal cord, brain and pineal complex &re involvd
in various degrees. Cyclostomes are light-sensitive but the reoeptorregions are not identical in different forms. Hagfish nnd lamprey8
respond to light by general or increased locomotory activity after a
rather lengthy reaction period (several seconds). The hag is practically
eyeless, its photoreceptors are located in the skin and occur mostly at
the anterior end of the head and in the cloaca1 region. Afferent impulsee
from the posterior photoreceptors travel through spinal nerves to the
spinal cord. Thence they proceed anteriorly to the brain, but local
reflexes can still be elicited after spinal section (Newth and Ross, 1955).
The ammocoetes (larva of the lamprey) is virtually blind whereas adults
have eyes. The caudal region is especially sensitive to light; photoreceptors occur in the skin and afferent pathways lie in the lateral line
nerve. The middle region of the body shows some photoeeneitivity, due
apparently to direct stimulation of the spinal cord by light, and there
is a photosensitive region in the head in the region of the pineal complex.
When the head of a lamprey is illuminated, its responses are lea8
regular and reaction times are longer than those elicited by stimulation
of the tail. It still responds to illumination of the head after removal
of the pineal and parapineal bodies and the paired eyes; there are
obviously other photoreceptors in the head, either in the skin or in the
brain (Young, 1935a).
The experimental approach just described involves detecting
sensitivity of photcmcctptors nrd ddmmining operative prtthwayf3. on
a different pltirw w! to h C O I I H J ~ ~ * ~
f h t kirrrlw of rwpori@~;~ svcikd by
different phc,torcceptora. lri thc. untrnococteR, for oxwn pie, illurninahion
of the tail initiate8 Hwimmirig niovcrnentx leading to burrowing ; in the
adult, simply swimming (negative photokinefh). When paired eye6 of
the adult lamprey (Lampetru) are illuminated, however, the oonsequences are uneasy movements of the animal and altered breathing
rates. Moreover, lampreys undergo colour-changes, controlled by
incident light. The colour-response di.sayJpears in the larva and is
disturbed in the adult after destruction of the pineal complex, when the
melanophores become expanded, a1 td it is completely abolished in the
adult by removal of the pineal complex and paired eyes. It seems,
therefore, that dermal photoreceptors of the tail are primarily responsible for generalized swimming movements (leading to burrowing in
larvae) ; the pineal organ controls mlour-responses in the larvae and
pineal plus paired eyes act synergetiaally in controlling colour-responses
J. A. C. NICOL
II. EXTRA-OCU- RECEPTION
Photoreception in many fishes may be mediated by structures other
than the eyes. Skin, spinal cord, brain and pineal complex &re involvd
in various degrees. Cyclostomes are light-sensitive but the reoeptorregions are not identical in different forms. Hagfish nnd lamprey8
respond to light by general or increased locomotory activity after a
rather lengthy reaction period (several seconds). The hag is practically
eyeless, its photoreceptors are located in the skin and occur mostly at
the anterior end of the head and in the cloaca1 region. Afferent impulsee
from the posterior photoreceptors travel through spinal nerves to the
spinal cord. Thence they proceed anteriorly to the brain, but local
reflexes can still be elicited after spinal section (Newth and Ross, 1955).
The ammocoetes (larva of the lamprey) is virtually blind whereas adults
have eyes. The caudal region is especially sensitive to light; photoreceptors occur in the skin and afferent pathways lie in the lateral line
nerve. The middle region of the body shows some photoeeneitivity, due
apparently to direct stimulation of the spinal cord by light, and there
is a photosensitive region in the head in the region of the pineal complex.
When the head of a lamprey is illuminated, its responses are lea8
regular and reaction times are longer than those elicited by stimulation
of the tail. It still responds to illumination of the head after removal
of the pineal and parapineal bodies and the paired eyes; there are
obviously other photoreceptors in the head, either in the skin or in the
brain (Young, 1935a).
The experimental approach just described involves detecting
sensitivity of photcmcctptors nrd ddmmining operative prtthwayf3. on
a different pltirw w! to h C O I I H J ~ ~ * ~
f h t kirrrlw of rwpori@~;~ svcikd by
different phc,torcceptora. lri thc. untrnococteR, for oxwn pie, illurninahion
of the tail initiate8 Hwimmirig niovcrnentx leading to burrowing ; in the
adult, simply swimming (negative photokinefh). When paired eye6 of
the adult lamprey (Lampetru) are illuminated, however, the oonsequences are uneasy movements of the animal and altered breathing
rates. Moreover, lampreys undergo colour-changes, controlled by
incident light. The colour-response di.sayJpears in the larva and is
disturbed in the adult after destruction of the pineal complex, when the
melanophores become expanded, a1 td it is completely abolished in the
adult by removal of the pineal complex and paired eyes. It seems,
therefore, that dermal photoreceptors of the tail are primarily responsible for generalized swimming movements (leading to burrowing in
larvae) ; the pineal organ controls mlour-responses in the larvae and
pineal plus paired eyes act synergetiaally in controlling colour-responses