Body symmetry alone is not a foolproof criterion for assigning an animal phylum to a particular evolutionary line. The radial symmetry of some animals has apparently evolved secondarily from a bilateral condition as an adaptation to a more sedentary lifestyle. For example, sea urchins (phylum Echinodermata) are radially symmetrical, but their embryonic development and internal anatomy show clearly that they arose from a bilaterally symmetrical ancestor and belong with the bilateria.