The many types of worms in the world are divided into three phyla: Flatworms belong to the phylum Platyhelminthes and are flat, elongated animals that have bilateral symmetry and primitive organs (they include the planarians, flukes, and tapeworms). The phylum Nematoda includes the roundworms, or unsegmented worms, also with bilateral symmetry, but have little in terms of sensory organs. Most of them are parasitic, including the parasite Trichinella that causes trichinosis from uncooked pork. The third phylum is Annellia, or segmented worms, including sandworms, tube worms, earthworms, and leeches. They also have bilateral symmetry, a digestive tract that is a tube within a tube—complete with a crop, gizzard, and intestine—and a closed circulatory system.