The phylum whose organisms are characterized by the presence of a dorsal hollow nerve cord and paired pharyngeal gill slits is Phylum Chordata. It is divided into three sub-phyla i.e. Urochordata, Cephalochordata and Vertebrata. Urochordates and Cephalochordates are together referred to as protochordates. Urochordates show the presence of notochord only in the larval tail. Cephalochordates show the presence of notochord from head to tail and is persistent throughout life. Myxine belongs to the class Cyclostomata. Its members show the presence of sucking circular mouth without jaws. The body is devoid of scales and paired fins. Members of the class Osteichthyes show the presence of an air bladder, a streamlined body and four pairs of gills covered by operculum on each side. Frog belongs to the Class Amphibia. The members of this class are cold blooded, show external fertilization and have a three chambered heart i.e. two auricles and one ventricle.