The most primitive animals, the sponges, have a distinct organization and body form, but lack tissues. So loosely is the body organized, that it can be filtered through a fine cloth and reassemble on the other side. From such a loosely organized ancestor, evolved an animal that had true tissues, a feature to be retained by all animals evolved thereafter. The cnidarians became predatory, two-layered bags with two specialized tissues. 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 flat worms added features such as cephalization, the possession of a head end that is the sensory center. Another innovation was 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. In the next chapter, the coelomate body plan and the organisms that have the plan will be studied in detail.