Acoelomates consist of two modern phyla of worms -- the Platyhelminthes and Nemerteans. These animals are quite common with perhaps 20,000 species known. Some are free living. Many are parasitic. Although the acoelomates are triploblastic with three germ layers in the developmental forms, they are developmentally more primitive than the coelomate phyla. They are therefore thought to have evolved prior to the coelomates (arthropods, chordates, etc). They are the most primitive forms with bilateral symmetry and having a head and internal organs. They have no body cavity. Their digestive systems are primitive at best -- a pit with a single opening rather than a tube with a mouth and anus. In many cases, nutrients are absorbed through the skin rather than eaten and digested.