Biology Reference
In-Depth Information
containing the light-sensitive protein. Interestingly, the adult box jellyfish also pos-
sesses such simple eyespots but in addition has true eyes, complete with lens, iris
and retina. The box jellyfish does not contain a brain, which tells us that a brain is
not necessary for eyes to evolve. It is common for individual invertebrate animals,
especially annelids and molluscs, to possess more than one type of light-detecting
structure.
A further elaboration of the simple eyespot would be for the shielding pigment
granules to occur in an adjacent cell to the one containing the light-sensitive protein-
pigment; this arrangement occurs in the larvae of the ragworm,
Platynereis
.Ifthe
number of light-sensitive cells shielded by the adjacent cells then increases, the
resulting structure can determine the light intensity from more than one direction
at the same time - this type of structure is found in the flatworms called planaria.
Planarians seek the shade, so use their eyespots to find dark environments where
they are safer from predators. All these stages are illustrated in Figure 5.2.
Fig. 5.2
The next step from a planarian eyespot would be for the structure to deepen
further into a pit and contain more light-sensitive cells, so that the direction of light
can be determined more precisely. Pit eyes are common in invertebrates, such as
the mollusc
Nautilus
(see Figure 5.3). The diameter of the opening in the
Nautilus
eye can be varied by a factor of seven-fold to accommodate varying intensities of
light. There are now two evolutionary possibilities: either the pit eye can develop
