8. VISION AND BIOLUMINESCENCE
235
side of the eye and the blood from the ommatidial sinuses appears
to gain access through this complex to the eye sinuses proper. The
outer and inner eye sinuses in the eye stalk join together at the base of
the eye and the blood passes from them to the cephalothoracic sinus
and so returns to the pericardium and heart.
There is a considerable amount of pigment in the eyes. A brownishblack pigment that remains in almost the same position, regardless of
the conditions of light or darkness to which the eye is exposed, is present
between the crystalline cones (Fig. 79). I n eyes exposed to light, there
is more pigment of the same colour forming a layer over the whole eye
at the level of the outer ends of the rhabdoms and also a bright orangered pigment that is concentrated between the inner ends of the rhabdoms
to form a layer a t their lower ends (Fig. 79). The orange-red pigment
withdraws from the rhabdoms to form a densely coloured band below
the basement membrane, causing the eye to be more sensitive to
prevailing conditions of very low light intensity.
The chemical nature of the brownish-bIack pigment has yet to be
determined. Fisher and Goldie (1958) reported that the black pigment,
mostly in the regions of the crystalline cones, rhabdoms and photophore,
was a melanin and not an ommochrome. They later (Fisher and Goldie,
1961) modified the view that it is not an ommochrome to a statement
that “ it is not an ommin ”. D. J. Pritchard (L. R. F.’s assistant)
repeated this work and concluded that the major dark pigment in these
regions is probably not melanin but may, in fact, be an ommochrome,
although a small quantity of melanin may also be present. His experimental results suggest that the histochemical tests for melanin are not
specific; the tests used were those of Stinson et al. (1959), Lillie (1957)
and the Bodian reaction as described by Dublin (1943). More work is
required before these pigments can be defined chemically. The orangered pigment is the carotenoid, astaxanthin.
It is obvious that such an eye is nearly as complicated as that of a
vertebrate. The vertebrate eye has a single lens with a liquid phase, the
aqueous humour, in front of it and underlying the cornea and a more
gelatinous phase, the vitreous humour, lying behind it and filling a
spherical cavity lined with nervous tissue outside of which is the photosensitive layer, the retina. There are in most vertebrates two kinds of
retinal elements, the rods and cones, from which nerve fibres connect
to the layer of nervous tissue whence all converge to a point, the blind
spot, where they pass through the retina and the outer layers, the
pigmented choroid and the supporting layer or sclerotic, to become the
optic nerve passing back to the optic lobe of the brain. The cones are
the elements concerned with normal daylight or photopic vision. The
235
side of the eye and the blood from the ommatidial sinuses appears
to gain access through this complex to the eye sinuses proper. The
outer and inner eye sinuses in the eye stalk join together at the base of
the eye and the blood passes from them to the cephalothoracic sinus
and so returns to the pericardium and heart.
There is a considerable amount of pigment in the eyes. A brownishblack pigment that remains in almost the same position, regardless of
the conditions of light or darkness to which the eye is exposed, is present
between the crystalline cones (Fig. 79). I n eyes exposed to light, there
is more pigment of the same colour forming a layer over the whole eye
at the level of the outer ends of the rhabdoms and also a bright orangered pigment that is concentrated between the inner ends of the rhabdoms
to form a layer a t their lower ends (Fig. 79). The orange-red pigment
withdraws from the rhabdoms to form a densely coloured band below
the basement membrane, causing the eye to be more sensitive to
prevailing conditions of very low light intensity.
The chemical nature of the brownish-bIack pigment has yet to be
determined. Fisher and Goldie (1958) reported that the black pigment,
mostly in the regions of the crystalline cones, rhabdoms and photophore,
was a melanin and not an ommochrome. They later (Fisher and Goldie,
1961) modified the view that it is not an ommochrome to a statement
that “ it is not an ommin ”. D. J. Pritchard (L. R. F.’s assistant)
repeated this work and concluded that the major dark pigment in these
regions is probably not melanin but may, in fact, be an ommochrome,
although a small quantity of melanin may also be present. His experimental results suggest that the histochemical tests for melanin are not
specific; the tests used were those of Stinson et al. (1959), Lillie (1957)
and the Bodian reaction as described by Dublin (1943). More work is
required before these pigments can be defined chemically. The orangered pigment is the carotenoid, astaxanthin.
It is obvious that such an eye is nearly as complicated as that of a
vertebrate. The vertebrate eye has a single lens with a liquid phase, the
aqueous humour, in front of it and underlying the cornea and a more
gelatinous phase, the vitreous humour, lying behind it and filling a
spherical cavity lined with nervous tissue outside of which is the photosensitive layer, the retina. There are in most vertebrates two kinds of
retinal elements, the rods and cones, from which nerve fibres connect
to the layer of nervous tissue whence all converge to a point, the blind
spot, where they pass through the retina and the outer layers, the
pigmented choroid and the supporting layer or sclerotic, to become the
optic nerve passing back to the optic lobe of the brain. The cones are
the elements concerned with normal daylight or photopic vision. The