Class Agnatha (jawless fish) And Subclasses
Class Agnatha is a superclass of jawless fish in the phylum Chordata , subphylum Vertebrata , consisting of both present ( cyclostomes ) and extinct ( conodonts and ostracoderms ) species. The group excludes all vertebrates with jaws, known as gnathostomes . The agnathans as a whole are paraphyletic, because most extinct agnathans belong to the stem group of gnathostomes. Recent molecular data, both from rRNA and from mtDNA as well as embryological data strongly supports the hypothesis that living agnathans, the cyclostomes, are monophyletic. The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, comprising about 120 species in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the group Craniata was created by Linnaeus (and is still sometimes used as a strictly morpho
