Biology Reference
In-Depth Information
into a larger structure containing more light-sensitive cells called a chambered eye,
or the entire pit eye can be duplicated many times to form a compound eye. The
essential distinction between these two forms is that there is only one optical system
in chambered eyes, but mutiple optical systems in the compound eye. The most
detailed images are formed by chambered eyes because the amount of detail possible
in the pit eye is limited by the diameter of the opening - the smaller the opening the
better the detail, but the less the amount of light that falls on the light-sensitive cells.
In chambered eyes this problem is overcome by the evolution of the lens, which
enables detailed images to be formed at low light intensities.
Chambered eyes are sometimes called simple eyes, but this is a poor name
because they are far from simple! The pit eye is initially open at the top to admit
light, but if this opening is covered by a transparent layer of cells to prevent the pit
being blocked by detritus or parasites, it allows the pit to be filled with transparent
liquid, which improves its optical properties. This in turn, allows the subsequent
development of internal devices that dramatically improve the quality of the image,
such as an iris, lens or mirror. The octopus has a lens-containing eye very sim-
ilar to the human eye, while the scallop
Pecten
has a mirror eye (Fig. 5.3). The
lens is made of concentric layers of highly elongated cells packed with transparent
proteins called crystallins. Analyses of the amino sequences of crystallins reveal
that they are closely related to ordinary proteins involved in metabolic pathways -
they have been co-opted for vision because they happen to have the right degree of
Fig. 5.3
