How do Red Brittle Stars...

Stars have sensory organs on the tips of each arm, along with a brain analog for each arm. This is why they can regrow from just one arm and a chunk of the central disk. They can sense light and dark, as well as some basic self preservation need (ie, this is food, this is not, light here, dark here) but the majority of their sensor input is from their senstive feeler/feet. The little tube feet provide lots of environmental information for them.

This reminds me of an article I came across in Nature a few years ago. Turns out, brittle stars have little calcite lenses all over their arms.

Calcitic microlenses as part of the photoreceptor system in brittlestars
Aizenberg J, Tkachenko A, Weiner S, Addadi L, Hendler G
NATURE
412 (6849): 819-822 AUG 23 2001


Abstract:
Photosensitivity in most echinoderms has been attributed to 'diffuse' dermal receptors(1-3). Here we report that certain single calcite crystals used by brittlestars for skeletal construction(4,5) are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma(6) showed that in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design. These structures are absent in light-indifferent species. Photolithographic experiments in which a photoresist film was illuminated through the lens array showed selective exposure of the photoresist under the lens centres. These results provide experimental evidence that the microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4-7 mum below the lenses) coincides with the location of nerve bundles-the presumed primary photoreceptors. The lens array is designed to minimize spherical aberration and birefringence and to detect light from a particular direction. The optical performance is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical functions.