Sea urchins use their entire body as an eye:
For decades, scientists knew that sea urchins can respond to light, even though they don’t have anything that looks remotely like an eye. The mystery deepened in 2006, when the full genome of the purple sea urchin was published. To everyone’s surprise, its 23,000 genes included several that are associated with eyes. The urchin has its own version of the master gene Pax6, which governs the development of animal eyes from humans to flies. It also has six genes for light-sensitive proteins called opsins.
While these genes are usually switched on in the developing eye, Maria Arnone found that the sea urchin’s versions are strongly activated in its feet. Sea urchins have hundreds of “tube feet”, small cylinders that sway around amid the spines. They can use the feet to move around, to manipulate food, and apparently to see.
Esther Ullrich-Luter – one of Arnone’s collaborators – found that each foot has two clusters of light-sensitive cells: one at the tip and another at its base. Each foot has up to 140 of these cells, giving a total of 200,000 across the entire animal. (For comparison, humans have a thousand times as many.)
The light-sensitive cells connect to a single nerve running down the length of each foot. The nerves of the tube feet eventually cluster into five spokes, which meet at a central ring of nerves. This is the extent of the urchin’s nervous system – it’s a sparse network of nerves without any central brain. Through this network, the sea urchin detects can react to light, which it spots with its hundreds of feet. Its entire surface is effectively a big compound eye.
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