ALL: D. KNOPthe light is coming. In the aquarium, we find pigment-
cup eyes in many bristleworms and snails, such as Patella
and Haliotis. A typical pigment-cup eye is the “nauplius
eye,” a characteristic feature of the development of crus-
taceans. It is so typical for these animals that a larva can
be identified as a crustacean larva by its presence. Many
crustaceans have it only in the larval stage (e.g. Artemia),
but others retain it in the adult stage (e.g. copepods).
The nauplius eye sits in the middle of the head (hence
the term “median eye”). It always occurs singly and al-
ways consists of three closely packed pigment-cup ocelli.
PINHOLE EYE
The pinhole eye is derived from
the pigment-cup eye, but in this
case the light entry opening is
constricted to form a small hole
so that the optical effect of an
aperture is produced. This gen-
erates a low-light image with the
light-sensing cells on the poste-
rior wall of the eye-cup (e.g. in
the Nautilus). However, this al-
lows only a rudimentary percep-
tion of the environment. The in-
terior of the eye is filled with surrounding water because
the light inlet opening is not closed. The Nautilus eye has
not developed further in 600 million years—change was
not necessary because of its way of life. During the day
this animal retreats from predators to lightless depths of
around 1,600 feet (500 m); at night it ascends to lower
reef zones to feed and spawn. A sharp lens eye would not
offer a survival advantage in either place.
However, this is not the case for all cephalopods,
which existed several hundred million years ago, because
their shells had two slightly different designs. NautilidsThe pinhole eye of
Nautilus pompilius.
Schematic representation of the
pinhole eye.Fossil remnants of two cephalopods whose
descendants have radically different eyes:
an orthocerid with a stretched shell and a
rolled-shell goniatite. The fossils are about
350 million years old.