Sea urchins don't have eyes in the strict sense, but they can detect light through thorns. Sea urchins know their surroundings by comparing the intensity of light. The greater the density of thorns, the sharper the spatial vision of sea urchins.

Sea urchins have five pairs of gills in the enclosure. Gill is a branch structure protruding from the body wall and is the main place for gas exchange. Its internal cavity is communicated with the body cavity around the pharynx and filled with body cavity fluid.

The bone fragments of Aristotle lanterns and the contraction pressure of muscles squeeze body fluids into gills for gas exchange. Its attractive movement is that the changes of oxygen and carbon dioxide in body cavity fluid stimulate nerves, and then nerves dominate the movement of bone fragments and muscles.

There is no special sensation, and the sensory cells are mainly distributed between the epithelial cells of the pins, spines and spines, which have the functions of touch and taste. There are also spherical corpuscles, which have a balancing effect. Sea urchins are also very sensitive to light, mostly negative phototaxis, and the reverse epidermal cells have eye spots or photoreceptor cells.

Living habits of sea urchins:

Born timid, he runs as long as he sees the enemy, but the sea urchin doesn't move fast. Most sea urchins live on the bottom of the sea, and they like to live in reef forests or crevices in the sea below the intertidal zone rich in algae, as well as in the hard sandy muddy shallow sea zone, which has the characteristics of avoiding light and coming out at night.

Sea urchins will move with food intake: if there is enough food, they can move more than 1 meter every day; If food is scarce, sea urchins may only move 10 cm every day. The movement of sea urchins is accomplished by transparent, small, numerous and sticky tube feet and spines.

The tube foot moves like a starfish, which can catch rocks, while the spines at the bottom hold up the sea urchin's body and help it move at will. When they move, they can always take the direction of the walking belt as the guide without turning their heads. When the sea urchin is turned over, its thorns and tube feet can make it turn over to the right.

Sea urchins have a wide range of foods. Carnivores will feed on worms, mollusks or other echinoderms on the seabed, while herbivores mainly feed on algae. In addition, there are sea urchins, which feed on organic debris and animal carcasses.