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Eyes are organs that detect light. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms and may allow the organism to seek out or avoid light. More complex optical systems with resolving power have come in ten fundamentally different "designs", and 96% of animal species possess a complex optical system.[1] Nature has produced ten different eye layouts - indeed every way of capturing an image has evolved at least once in nature, with the exception of zoom and Fresnel lenses. Eye types can be categorised into "simple eyes", with one concave chamber, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface.[1] Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution - the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposotion eyes, so are better suited to dark-dwelling creatures.[1] Eyes also fall into two groups on the basis of their photoreceptors cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomic. These two groups are not monophyletic; the cnidaira also posess cilliated cells, [6] and some annelids possess both.[7]