Brittle stars, snake stars, and ophiuroids are echinoderms related to starfish in the class Ophiuroidea. They move across the sea floor by crawling with their flexible arms. The Ophiuroidea is divided into two major clades, ophiurida and Euryalida. Today, ophiuroids can be found in all major marine provinces, from the poles to the tropics. Brittle stars are also abundant in reef habitats, where they can be found hiding under rocks and even within other living species. A few ophiuroid species can even endure brackish water, which is nearly unheard of in echinoderms. The skeleton of a brittle star is made up of embedded ossicles. The Ophiuroidea may have the strongest inclination toward five-segment radial symmetry of any echinoderm. The body form of ophiuroids is similar to that of starfish, with five limbs connected to a central body disc. In ophiuroids, however, the central body disc is clearly separated from the arms. All of the viscera are contained on the disc. That is, unlike the Asteroidea, the internal organs of digesting and reproduction never enter the arms. The mouth is located on the underside of the disc and is made up of five toothed jaws made of skeletal plates. The madreporite is normally found within one of the jaw plates, rather than on the animal's upper side, as it is in starfish.