Müller & Troschel, 1842
Brittle stars have long, thin arms emanating from a small, disk-shaped body and are about the size of an outstretched human hand. They belong to the phylum of echinoderms, which includes sea urchins, sea cucumbers, and sea stars.
Visual system 
Its arms are covered with calcite crystals. In addition to functioning as an armor and giving structural support, the crystals make up its unique visual systems. They minimize spherical aberration and can detect the direction of incoming light. The lenses work by filtering and focusing light on an underlying photoreceptor system. Nerve bundles under each lens, presumed to be light-sensitive, transmit the optical information to the rest of the nervous system.
The only known animals to employ a similar visual system were the now-extinct trilobites. Phototropic chromatophores can change O. wendtii's color and regulate how much light will reach the photoreceptors.
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