The Sea Urchin
Sea Urchins have no brain, and have a relatively simple nervous system layout. The center is a large nerve ring that encompasses the mouth just inside the urchin's lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system and branch into even finer nerves to innervate the tube feet, spines, and the pedicellariae, which is a kind of defensive organ. Sea urchins are sensitive to touch, light, and chemicals, and even though they have no eyes or eyespots, studies show that their whole body might work as one compound eye.