had a rolled shell that was snail-shaped, allowing them
to descend easily to great depths and access flat water
zones without physical exertion. Members of the order
Orthocerida had a straight, elongated housing. This
shape was less suitable for diving, so they had to spend
day and night in their shallow-water habitat. In order to
survive, they developed lens eyes and became nimble and
intelligent. The octopuses (Octopoda) and cuttlefishes
(Sepiida) developed from these creatures. This example
shows how much environmental requirements affect the
evolution of the eye.
BUBBLE EYE
The bubble eye is derived from the pinhole eye. The vi-
sual opening is not only narrowed to a hole, but also
covered by a transparent membrane, forming a bubble.
The inside of this eye is filled with a transparent
secretion that has a refractive function, so it can
be called a primitive lens. The light that comes
through this opening projects a blurred image on
the retina. The smaller the aperture, the sharper
the image, but it is correspondingly dimmer. In the
aquarium many snails have bubble eyes.
COMPOUND EYE
Compound eyes occur in arthropods. They con-
sist of numerous individual elements called om-
matids. Together they form a honeycomb pattern
and are usually arranged hemispherically. Each of
these ommatids produces its own partial image,
and the totality of these images allows the ani-
mal to visualize the environment. The number of
ommatids ranges from just a few—for example, in
Lepisma (e.g. silverfish) and Prociphilus (aphids) to
almost 30,000 per eye in large dragonflies. Mantis
shrimps have up to 10,000 ommatids per eye. The
ommatids that are directed at our viewing eye (or
camera lens) appear darker than the surrounding
ones. In the marine aquarium, we encounter com-
pound eyes in all crustaceans, including shrimps,
round crabs, and mantis shrimps.
The resolution offered by the compound eye is
inferior to that of the lens eye. However, the com-
pound eyes of some animals can perceive move-
ments much faster than lens eyes. While the human
eye is able to perceive a maximum of 25 successive light
flashes per second, some compound eyes can detect up to
300 light flashes per second.THE EYES OF MANTIS SHRIMPS
The compound eyes play a special role in mantis shrimps
(Stomatopoda). While other animals use two to four dif-
ferent receptor types or visual pigments on average, and
humans have three, mantis shrimps have no fewer than
twelve. However, the high number of different visual
pigments has led to a misinterpretation that is still very
common. We were convinced that these animals have
fantastic vision and a much more diverse color percep-
tion than humans. However, as Thoen and colleagues
(2014) recently found, this is not the case.Ocelli of Turbo petholatus and
schematic representation.The compound eye of Odontodactylus
scyllarus: The ommatids, which appear black,
focus on the observer from two different
points, so the single eye provides three-
dimensional images and allows the estimation
of distances. The intervening ommatidial band
is responsible for color perception.