Simplify the following text for a Grade 15 native English-speaking person.
Original: Craniata ( sometimes Craniota ) is a proposed clade of chordate animals that contains the Myxini ( hagfish ) , Petromyzontida ( including lampreys ) , and Gnathostomata ( jawed vertebrates ) as living representatives .
Simplified: Craniata ( sometimes Craniota ) is a proposed clade of chordate animals that contains the vertebrates ( subphylum Vertebrata ) and Myxini ( hagfish ) as living representatives .