Sea urchins have mouths. Although the body structure of sea urchins is very simple, with only round bodies and long thorns, they still have mouths. Sea urchins can't eat because they don't have a mouth. But the mouth of sea urchins is not obvious, and it is difficult to see them when they are not eating. Only when they eat can they observe their mouths. Under normal circumstances, the mouth of a sea urchin will have a white part on the surface where there are no thorns, and that is the mouth of a sea urchin.

Sea urchins have no eyes, they are echinoderms, and their body structure is relatively simple. Unlike other animals, they have no eyes. Although they don't have eyes, they have similar organs that can sense their surroundings, which is the result of sea urchins' evolution over the years. For example, there are some epidermal cells on the reverse surface of sea urchins, which have eye spots and can sense external light. In addition, there are enough photoreceptor cells on their spines and tube feet, so sea urchins are very sensitive to the surrounding light even if they have no eyes.

Sea urchins have no legs, their shape is round, and they are supported by thorns all over their body. Although these thorns are fragile, they can still protect sea urchins. In addition, these spines can also be used as legs when sea urchins want to move. Although sea urchins' spines can be used as legs, they move slowly.