The group of animals known as pelagic tunicates are animals in the Class Thaliacea. This class is divided into 3 orders: the Salipida (the salps), Doliolida, and the Pyrosomida. The Pyrosomes are strictly colonial, while the salps and doliolids alternate between colonial and solitary stages.
Animals in this class are usually barrel-shaped, gelatinous, clear planktonic organisms that are found in the water column. They are free-swimming, and move by pulling water in through a siphon at their front end, and forcing it out the back end using muscles and cilia.
Tunicates are relatives of vertebrate animals, because they have a notochord, pharyngeal gill slits, and a dorsal nerve cord, present during their larval stage, which allows them to be categorized in the Phylum Chordata, which also includes more familiar marine creatures like fish, sharks and cetaceans.