Cyclostome

The cyclostomes (or cyclostomi) are the lamprey and hagfish. They are the only two living jawless fish, the Agnatha.

Recent molecular data from rRNA,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.  and from mtDNA,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.  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" and "Cyclostomi" means "round mouths".Kuraku, Shigehiro, et al 2009. Jawless fishes (Cyclostomata). In S.B. Hedges & S. Kumar. Timetree of life. Oxford University Press. pp. 317–319.  Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.

 References 

Category:Agnatha
Category:Fish
Category:Vertebrates