Tunicata
. Tunicate, common name for any member of a subphylum of primitive marine chordates . There are about 2000 known species. The larval stage is characterized by the presence of a notochord and a dorsal nerve cord, both of which are lacking in the adult. The adult is characterized by a layer of protective secretion called a tunic. Tunicates are common in such habitats as rocky shores but can also be found at great depths. Species known as sea squirts make up one class in the subphylum, and most tunicates belong to this class. Tunicates are important because they are an evolutionary link between invertebrates and vertebrates. The larval stage, which generally resembles a tadpole, has a dorsal nerve cord protected by a notochord, a brain , gills, blood vessels, and a coelom. These features of tunicate larvae indicate that they are more closely related to the chordates than to the invertebrates . The free-swimming larva eventually attaches itself to a substrate and undergoes metamorphosis i