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An eyespot may be a specialized part of a cell as in protozoans, a single
photoreceptor cell, or a small cluster of receptors with few or no accessory optical
and neural structures. The entire epithelium (skin) of the annelid earthworm
Although the ctenophorans (comb jellies) usually have been assumed to lack
both a response to light and photoreceptor cells, recent evidence has raised
some doubt about such a conclusion. Electron microscopic examination of ...
photoreceptor, responds to a decrease in illumination; the proximal retina, with a
microvillar photoreceptor, responds to increasing illumination. The cephalopod
retina has long cylindrical photoreceptor cells with rhabdomeres consisting of ...
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For virtually all its 238 years,Britannica has ruled the shelves as a peerless general encyclopedia. Reviewers have consistently given it high marks for most of the 12 criteria identified inKister's ... Read full review
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