The eyes on sea urchins are anal mastoid, which is not eyes, but the excretion hole of sea urchins. Sea urchins discharge waste from their bodies through anal mastoid. Although sea urchins have no eyes, they have many auxiliary organs to sense the external environment. For example, there are some epidermal cells on their reverse surface, which can help them sense external light. In addition, there are many photoreceptor cells in their spines and foot tubes, which are enough for them to survive and move. This shows that the more thorns on sea urchins, the more sensitive they are to light.

Sea urchins have no legs. They are echinoderms and belong to a relatively lower animal. Sea urchins are simple in structure. They are round and have many spines on their shells. Besides, they have no other organs, so sea urchins have no legs. Although they have no legs, they can use thorns to change their position. When they move, they slowly adjust the position of thorns so that they can roll like balls, so thorns can also be counted as sea urchin legs. Of course, the spines of sea urchins are different from the legs of other animals. They can't make sea urchins move quickly, but they can only move slowly.