Subphylum Urochordata consists of all tunicates. Tunicates are invertebrates, lacking a backbone consisting of vertebrae. Adult tunicates look like small sacs (about 3 cm tall) and are stationary, lacking a nerve cord, a notochord, and a post-anal tail. However they do have very large gill structures, which allows the passage of water out of the adult. Water enters through a hole at the top of the tunicate, and organic particles are trapped in the mucus near the gills, and digested in the intestine. Lacking three of the four distinguishing hallmarks of the chordates, it would seem impossible for these animals to be placed in phylum Chordata. However, tunicate larvae have a post-anal tail, a nerve cord, and a notochord. Therefore, these immobile animals with tadpolelike larvae are considered chordates. There are approximately 1,600 species in the subphylum, which also consists of sea squirts and salps.