Many zoologists believe that the acoelomate body plan is the basis for the other invertebrate phyla.
The free-living flatworms or planarians are aquatic bottom dwellers, both scavengers and predators. A single-opening gut is highly branched to maximize surface area and minimize distance for diffusion. The mouth is ventral, about halfway down, with a muscular pharynx to protrude into prey, soften up food with digestive enzymes, and tear it apart with sucking movements. Planarians are notorious for their abilities of regeneration. Single opening digestive tracts as with flatworms, but not as branched.