Simple eyes, called pigment pits or ocelli, are found in many invertebrates like jellyfish, worms, and snails. They contain a single lens but lack an elaborate retina. More complex eyes have distinct retinal, lens and cornea structures, like those found in box jellyfish. The development of simple and compound eyes in insects is genetically controlled by homeobox genes like orthodenticle and dachshund which pattern the eye segments and influence opsin expression.
2. Simple and Compound eye
• A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed
of a single lens and without an elaborate retina such as occurs in most Vertebrate.
• Some jellyfish, sea stars, flatworms, and ribbon wormsbear the simplest eyes, pigment spot ocelli, which
have pigment distributed randomly and which have no additional structures such as acornea and lens.
The apparent eye color in these animals is therefore red or black. However, other cnidaria have more
complex eyes, including those of Cubomedusae which have distinct retina, lens, and cornea.
• Many snails and slugs (gastropod molluscs) also have ocelli, either at the tips or at the bases of the
tentacles. However, some other gastropods, such as the Strombidae, have much more sophisticated eyes.
Giant clams (Tridacna) have ocelli that allow light to penetrate their mantles.
• The ocelli are simple eyes, meaning they collect and focus light through a single lens. These simple
eyes assist bees with sun orientation so they can navigate well during the day.
GENETIC CONTROL:
• The head patterning is controlled by orthodenticle, a homeobox gene which demarcates the segments
from the top-middle of the head to the more lateral aspects. The ocelli are in an orthodenticle-rich area,
and the gene is not expressed by the time one gets as lateral as the compound eyes.
• The gene dachshund is involved in the development of the compound eye.Different opsins are used in the
ocelli of compound eyes.
