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.
Its arms are covered with calcite crystals. In addition to functioning as an armor and giving structural support, the crystals were, until recently, thought to form a visual system. They minimize spherical aberration of incoming light and have excellent optical properties. The lenses were suggested to work by filtering and focusing light on an underlying photoreceptor system. Nerve bundles under each lens, presumed to be light-sensitive, would transmit the optical information to the rest of the nervous system. However, the discovery of nerves and photoreceptor cells in between, rather than beneath, the lenses suggests that this system may not rely on their optical properties.
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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