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Simple eye in invertebrates
A simple eye (sometimes called a pigment pit〔http://www.mendeley.com/research/structure-and-optics-of-the-eyes-of-the-box-jellyfish-chiropsella-bronzie/〕) refers to a type of eye form or optical arrangement that contains a single lens. A "simple eye" is so called in distinction from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word. The eyes of humans and large animals, and camera lenses are classed as "simple" because in both cases a single lens collects and focuses light onto the retina or film. Many insects have compound eyes consisting of multiple lenses (up to tens of thousands), each focusing light onto a small number of retinula cells.
The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, an enormous variety of eye designs are found in arthropods: they possess a wide variety of novel solutions to overcome visual problems or limitations.
==Ocelli or eye spots==
Some jellyfish, sea stars, and flatworms bear the simplest eyes, pigment spot ocelli, which have pigment distributed randomly and which have no additional structures such as a cornea 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.〔(【引用サイトリンク】title=Photoreceptors of cnidarians )〕
Many snails and slugs (gastropod mollusks) 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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