As adults, the anatomy of sessile sea squirts and their planktonic relatives does not resemble that of chordates. Nevertheless, they, and especially their larvae, possess several characteristic features that morphologically identify them as members of the chordate taxon. Recent molecular data have shown that the urochordates are the closest relatives (sister group) of vertebrates. The free-swimming tadpole larvae develop a notochord, a hollow dorsal nerve cord, and a swimming tail, which are critical characteristics of chordates. However, the adult sea squirts lose all these structures during metamorphosis and transform into sessile animals. All known 3000 species of tunicates are marine and are present in both the plankton and the benthos.