Worms are invertebrates, which means they do not have a backbone. They belong to several different phyla, including Annelida (segmented worms), Platyhelminthes (flatworms), Nematoda (roundworms), Sipuncula (peanut worms), Echiura (spoonworms), Acanthocephala (spiny-headed worms), Pogonophora (beardworms), and Chaetognatha (arrowworms) . The term "worm" is also used for the amphibian caecilians and the slowworm Anguis, a legless burrowing lizard.