The asymmetry of the sponges gave way to radial symmetry in the cnidarians. The cnidarians were also able to digest larger prey with their extracellular digestion. The digestive cavity had a double duty opening. Of the food taken in, that which could not be digested and absorbed was ejected through the same opening it entered. This feature was retained by the next higher organism, the flatworms.
Flatworms have their digestive cavity in a bilaterally symmetrical body.
The flatworms added features such as cephalization, the possession of a head end that is the sensory center and a body cavity. The body cavity allowed for expansion of the digestive tube and provided hydrostatic skeletons. Both flatworms and roundworms have parasitic members. The parasites have a reduced array of anatomical features, but are as highly evolved as the free-living relatives. The parasites are as well suited to their life style as are their free-living kin.