The common colonial tunicate, Aplidium sp.
The Phylum Chordata is named for the stiffened rod that extends down the back, called a notochord. Chordates have segmented external muscles, a an aorta below the notochord and a nerve cord above the notochord. In vertebrates the notochord is replaced past the embryo stage by a vertebral column which is usually made out of bone.
Its somewhat hard to see why sea squirts or tunicates are chordates. As adults they are solitary to colonial filter feeders, lacking a notochord and nerve cord. However, larval forms have a tail, a nerve cord and notochord. Like barnacles among the arthropods, many adult groups of sea squirts have attached themselves to the bottom by their head and taken up filter feeding, surrounding themselves with a tough protective coating, in this case, a cellulose tunic.