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, 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" and "Cyclostomi" means "round mouths". Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.