8. VISION AND BIOLUMINESCENCE
231
is adjacent to the rhabdom in adult crabs, the zoea larva of the lobster,
and the megalops larva of P. planipes. I n adult P. planipes and
euphausiids, the retinular cells increase in length instead of the cone
cells increasing in length to form a cone stalk (Fig. 79). Six of the
retinular cells have their nuclei distally among the crystalline cones but
the seventh cell, the " eccentric retinular cell '), is short, its nucleus
being near the outer end of the rhabdom in the position occupied by all
the retinular cells in lobsters and crabs. During development of the
eye, the retinular cells, as they grow in length, first lay down the rhabdom and later secrete a slender hyaline filament which, a t its distal
end, forms a cup round the base of the crystalline cone. Chun (1896)
describes the rhabdoms of euphausiid eyes as columns of plates
separated from each other by constrictions. Kampa (1965), however,
confirms VaissiBre's (1961) description of them as spirals, the degree of
tightness of the spiral varying between species and also between lobes
of bilobed eyes. Kampa found that the loosest spiral is present in the
spherical eyes of Meganyctiphanes norvegica, Nyctiphanes simplex, and
Euphausia pacifica while the spirals are much tighter in the rhabdoms
of the upper lobes of the bilobed eyes of Nematoscelis dificilis, Nematobrachion jlexipes, Stylocheiron maximum, and S. afine. On the outer
side of the eye, adjacent to the eye stalk, is a photophore, the structure
of which will be described later. The most important feature of the
photophore in relation to the eye is the dense layer of black and red
pigments which surrounds it and separates it entirely from the rest of
the eye, so preventing any light from that organ reaching the lightsensitive elements of the eye.
A double eye, such as that of Stylocheiron suhmii (Fig. 80D), is very
different from a spherical eye such as that of an Euphausia species
(Fig. SO). I n the spherical eye the cones are all of the same size whereas
in the double eye the cones of the upper or front eye are frequently
greatly enlarged relative to those in the lower eye. The layer immediately below the basement membrane in both spherical and double
eyes has a very rich blood supply, a feature common to all crustacean
compound eyes (Mauchline, 1958b). The median aorta cephaka passes
anteriorly and ventrally from the heart supplying, by branches, part
of the stomach, the cephalothoracic blood gland, the brain, and the
antennules. The main artery divides at the bases of the eye stalks to
form paired optic arteries. The optic artery passes up the eye stalk and
supplies the photophore with a branch which divides a t the frontal
edge of the photophore to produce numerous branches which ramify
down through the " posterior cellular layer ') and the outer edges of the
striated body (Fig. 81). The blood from these branches passes to the
231
is adjacent to the rhabdom in adult crabs, the zoea larva of the lobster,
and the megalops larva of P. planipes. I n adult P. planipes and
euphausiids, the retinular cells increase in length instead of the cone
cells increasing in length to form a cone stalk (Fig. 79). Six of the
retinular cells have their nuclei distally among the crystalline cones but
the seventh cell, the " eccentric retinular cell '), is short, its nucleus
being near the outer end of the rhabdom in the position occupied by all
the retinular cells in lobsters and crabs. During development of the
eye, the retinular cells, as they grow in length, first lay down the rhabdom and later secrete a slender hyaline filament which, a t its distal
end, forms a cup round the base of the crystalline cone. Chun (1896)
describes the rhabdoms of euphausiid eyes as columns of plates
separated from each other by constrictions. Kampa (1965), however,
confirms VaissiBre's (1961) description of them as spirals, the degree of
tightness of the spiral varying between species and also between lobes
of bilobed eyes. Kampa found that the loosest spiral is present in the
spherical eyes of Meganyctiphanes norvegica, Nyctiphanes simplex, and
Euphausia pacifica while the spirals are much tighter in the rhabdoms
of the upper lobes of the bilobed eyes of Nematoscelis dificilis, Nematobrachion jlexipes, Stylocheiron maximum, and S. afine. On the outer
side of the eye, adjacent to the eye stalk, is a photophore, the structure
of which will be described later. The most important feature of the
photophore in relation to the eye is the dense layer of black and red
pigments which surrounds it and separates it entirely from the rest of
the eye, so preventing any light from that organ reaching the lightsensitive elements of the eye.
A double eye, such as that of Stylocheiron suhmii (Fig. 80D), is very
different from a spherical eye such as that of an Euphausia species
(Fig. SO). I n the spherical eye the cones are all of the same size whereas
in the double eye the cones of the upper or front eye are frequently
greatly enlarged relative to those in the lower eye. The layer immediately below the basement membrane in both spherical and double
eyes has a very rich blood supply, a feature common to all crustacean
compound eyes (Mauchline, 1958b). The median aorta cephaka passes
anteriorly and ventrally from the heart supplying, by branches, part
of the stomach, the cephalothoracic blood gland, the brain, and the
antennules. The main artery divides at the bases of the eye stalks to
form paired optic arteries. The optic artery passes up the eye stalk and
supplies the photophore with a branch which divides a t the frontal
edge of the photophore to produce numerous branches which ramify
down through the " posterior cellular layer ') and the outer edges of the
striated body (Fig. 81). The blood from these branches passes to the