Agnathan Temporal range: Cambrian – Recent
Lampetra fluviatilis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Superclass:	Agnatha Cope, 1889

Ostracoderms from the group Osteostraci

Reconstruction of the mid-Devonian agnathan Neeyambaspis enigmatica

The Agnatha is a superclass of fish. It is paraphyletic. This means it is a convenient catch-all term, which does not follow the rules of cladistics.

For example, most extinct agnathans belong to the stem group (ancestral group) of gnathostomes. But according to the rules, one sister group should not contain ancestors of another sister group.

The living Agnatha (lampreys and hagfish) are known as cyclostomes. Recent molecular data from rRNA, and from mtDNA, show that these living agnathans are monophyletic. There are about 100 species. Hagfish are vertebrates but do not have vertebrae. It is believed that they lost their vertebrae during their lifestyle adaptations.