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. The skin of flukes and tapeworms is called a syncytial epithelium or tegument . In other words, the epidermis is not separated into distinct cells, but consists of a layer of absorptive cytoplasm on the outside and the nuclei underneath muscle layers on the inside. From them arise tadpole-like cercariae, which leave snails to seek the next host. Clonorchis sinensis , the Chinese liver fluke .