What is Eyespot?
[[File:Euglena - schema.svg|thumb|Schematic representation of a ''[[Euglena]]'' cell with red eyespot]] [[File:Chlamydomonas.svg|thumb|Schematic representation of a ''[[Chlamydomonas]]'' cell with chloroplast eyespot]] An '''eyespot''' is a simple [[organ (anatomy)|organ]] in biology used to detect light. It may be called an '''ocellus''', or '''pigment pit'''. They are quite common in small, simple invertebrates, such as ''[[Planaria]]''. They do not have lenses or any means of [[focus]]sing. Therefore, they can sense light from dark, but do not give the animal a visual scene as our eyes do.<ref>Land M.F. & Fernald R.D. 1992. The evolution of eyes. ''Annual Review of Neuroscience'' '''15''': 1–29. [http://www.annualreviews.org/doi/abs/10.1146/annurev.ne.15.030192.000245] {{Webarchive|url=https://web.archive.org/web/20160530112215/http://www.annualreviews.org/doi/abs/10.1146/annurev.ne.15.030192.000245 |date=2016-05-30 }}</ref> In many simple animals the eyespot senses light with a pigment molecule called an [[opsin]].<ref>Plachetzki D.; Fong C. & Oakley T. 2010. The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway. ''Proceedings of the Royal Society / Biological sciences'' '''277''' (1690): 1963–1969. doi:10.1098/rspb.2009.1797. PMID 20219739.</ref><ref>Shichida Y. & Matsuyama T. 2009. Evolution of opsins and phototransduction. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2781858/?tool=pmcentrez] ''Philosophical transactions of the Royal Society of London. Series B, Biological sciences''. '''364''' (1531): 2881–2895. doi:10.1098/rstb.2009.0051. PMID 19720651</ref> Then a [[nerve fibre]] goes from the eyespot to the simple nervous system. This allows the animal to move in response to (for example) a [[shadow]] passing over it. Eyespots also occur in single-celled [[protist]]s like ''[[Euglena]]'' and ''[[Chlamydomonas]]'', where it is connected to the [[flagellum]]. This allows the information from the light patch to influence the cells's movement. The first fossils of eyes th