Echinoderms

That of thorns on the skin is one of the characteristics of this group, exclusively marine and that I reach a great development in last epochs. The adults present symmetry of five radioes, though in some groups they can modify it. In the body of the adults a backbone is observed, that it departs from the mouth, from that they think up five radiuses provided with elements ambulacrales.In some groups, since for example, the starfishes, these radiuses are the arms of the animal. Nevertheless there are others (since it is the case of the irregular hedgehogs) in that this symmetry pentarradial is modified.

The sea urchin has the body covered with mobile thorns with which it can move slowly on the bottom, using him at the same time as defensive weapon opposite to the predators.