Echinoderms diversity: variations on a theme
Five-fold symmetry and hydrostatically operated tube feet also occur in the starfish and the brittle stars, however their body plan has become inverted and the mouth is on the undersides. Yet in another group of echinoderms the five-fold symmetry is less conspicuous and the body plan is elongated with a mouth and anus at the two ends. At the mouth the tube feet have become modified into tentacles which filter fine food particles. The five-fold symmetry and hydrostatic mechanisms did not develop further and the group is generally considered to be an evolutionary cul-de-sac.