8. VISION AND BIOLUMINESCENCE
229
phore present in each eye stalk. Consequently, this species is an exception because all other species have well-developed eyes and a photophore
present in each eye stalk. There are several species in which the diameter
of the widest part of the eye is about one sixth of the length of the body,
the eyes of these species thus being among the largest in the " stalk-eyed
Crustacea". Variation in the external morphology of the eyes of
different species is very noticeable and divided, as opposed to spherical,
eyes are present in species of the genera Thysanoessa, Stylocheiron,
Tessarabrachion, Nematoscelis, and Nematobrachion.
Euphausiids usually live a t depths exceeding 100m where there
is very little direct penetration of light. The sea water with its dissolved
organic substances acts as a differential filter, absorbing most of the
ultra-violet light near the surface. The red end of the spectrum is also
absorbed before the rays have penetrated to any depth. The part of
the spectrum which reaches farthest into the water is the blue green
light and so any light coming from above to most euphausiids will be
of this colour. I n addition, there is light originating in the sea around
the euphausiids because they and many other organisms produce light,
this bioluminescence also being of a blue green colour. This fact has to
be remembered when discussing vision and the photophores. Kampa
et al. (1959), by using lights of different colours, have shown that the
euphausiid eye is indeed most sensitive to blue green light, no matter
whether the intensity of the different colours of light was varied to give
an equal biological response or whether equal energies of illumination
were used.
Each compound eye is covered by a network of hexagonal facets,
giving it the appearance of a honeycomb with a curved surface, and
comprises many individual ommatidia. Each ommatidium is a narrow
cylindrical structure, bounded a t its distal end by the cornea (Fig. 79,
cor.) and below it the corneagenous cells and the cone cells. Next is the
large crystalline cone. Reside the cone is a group of seven long, narrow
cells, the distal pigment cells ; beneath them are seven retinular cells,
which surround a cylindrical structure with transverse striations,
known as the rhabdom and which is connected to the crystalline cone
by the hyaline filament (Kampa, 1965). At the lower ends of the
retinular cells is a transverse basement membrane which is continuous
over the whole of the inner part of the eye. Nerve fibres pass from the
retinular layer through this membrane to the mass of nervous tissue
which passes back, as the optic nerve, through the eye stalk to the
brain.
Kampa (1965) has compared the structure of the ommatidia of
euphausiids with those of other crustaceans. The eyes of euphausiids
229
phore present in each eye stalk. Consequently, this species is an exception because all other species have well-developed eyes and a photophore
present in each eye stalk. There are several species in which the diameter
of the widest part of the eye is about one sixth of the length of the body,
the eyes of these species thus being among the largest in the " stalk-eyed
Crustacea". Variation in the external morphology of the eyes of
different species is very noticeable and divided, as opposed to spherical,
eyes are present in species of the genera Thysanoessa, Stylocheiron,
Tessarabrachion, Nematoscelis, and Nematobrachion.
Euphausiids usually live a t depths exceeding 100m where there
is very little direct penetration of light. The sea water with its dissolved
organic substances acts as a differential filter, absorbing most of the
ultra-violet light near the surface. The red end of the spectrum is also
absorbed before the rays have penetrated to any depth. The part of
the spectrum which reaches farthest into the water is the blue green
light and so any light coming from above to most euphausiids will be
of this colour. I n addition, there is light originating in the sea around
the euphausiids because they and many other organisms produce light,
this bioluminescence also being of a blue green colour. This fact has to
be remembered when discussing vision and the photophores. Kampa
et al. (1959), by using lights of different colours, have shown that the
euphausiid eye is indeed most sensitive to blue green light, no matter
whether the intensity of the different colours of light was varied to give
an equal biological response or whether equal energies of illumination
were used.
Each compound eye is covered by a network of hexagonal facets,
giving it the appearance of a honeycomb with a curved surface, and
comprises many individual ommatidia. Each ommatidium is a narrow
cylindrical structure, bounded a t its distal end by the cornea (Fig. 79,
cor.) and below it the corneagenous cells and the cone cells. Next is the
large crystalline cone. Reside the cone is a group of seven long, narrow
cells, the distal pigment cells ; beneath them are seven retinular cells,
which surround a cylindrical structure with transverse striations,
known as the rhabdom and which is connected to the crystalline cone
by the hyaline filament (Kampa, 1965). At the lower ends of the
retinular cells is a transverse basement membrane which is continuous
over the whole of the inner part of the eye. Nerve fibres pass from the
retinular layer through this membrane to the mass of nervous tissue
which passes back, as the optic nerve, through the eye stalk to the
brain.
Kampa (1965) has compared the structure of the ommatidia of
euphausiids with those of other crustaceans. The eyes of euphausiids