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.
The name Cyclostomata means "round mouths". Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.
- Mallatt J. & Sullivan J. 1998. 28S and 18S ribosomal DNA sequences support the monophyly of lampreys and hagfishes. Molecular Biology and Evolution 15 (12): 1706–1718. 
- DeLarbre, Christiane et al 2002. Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: the comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. Molecular phylogenetics and evolution. 22, 2, 184–192. 
- Kuraku, Shigehiro, et al 2009. Jawless fishes (Cyclostomata). In S.B. Hedges & S. Kumar. Timetree of life. Oxford University Press. pp. 317–319. ISBN 978-0-19-953503-3