Cyclostomi, often referred to as Cyclostomata /sɪkloʊˈstɒmətə/, is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes.

Both groups have jawless mouths with horny epidermal structures that function as teeth called ceratodontes, and branchial arches that are internally positioned instead of external as in the related jawed fishes.

Most fossil agnathans, such as galeaspids, thelodonts, and osteostracans, are more closely related to vertebrates with jaws (called gnathostomes) than to cyclostomes.

The "cyclostome hypothesis", on the other hand, holds that lampreys and hagfishes are more closely related, making cyclostomi monophyletic.

Similarities in the cartilage and muscles of the tongue apparatus also provide evidence of sister-group relationship between lampreys and hagfishes.

[14] The embryonic development of hagfishes was once held to be drastically different from that of lampreys and gnathostomes, but recent evidence suggests that it is more similar than previously thought, which may remove an obstacle to the cyclostome hypothesis.

Lamprey larvae live in freshwater and are called ammocoetes, and are the only vertebrates with an endostyle, an organ used for filter feeding that is otherwise found only in tunicates and lancelets.

It has been suggested that the hagfish ancestors evolved from an anadromous or freshwater species that has since adapted to saltwater over a very long time, resulting in higher electrolyte levels in its blood.

Because ciliated intestines are also found in Chondrostei, lungfishes and the early stages of some teleosts, it is considered a primitive condition that has been lost in hagfishes.