The term acoelomates means "without coeloms", the coelom being a body space surrounded by an extra layer of cells, the mesoderm. The most important acoelomates are the flat worms contained in the phylum Platyhelminthes, including the free-living Turbellaria, the Trematoda or liver flukes and the Cestoda or parasitic tapeworms. Other phyla included are Nemertina, the ribbon worms, Orthonectida, Rhombozoa and Gnathostomulida, members of which are typically worm-like creatures sometimes classed together as polychaetes, or bristly worms. Classification in this area of the animal kingdom is unclear.