Cyclostomi

Cyclostomi is a clade of jawless vertebrates comprising hagfishes and lampreys. Its members lack paired fins and true jaws, and have a circular mouth adapted for feeding.

Vertebrate: An animal with a backbone or a notochord that persists into adulthood. Cyclostomes are vertebrates whose evolutionary placement depends on how vertebrate traits are defined.
Hagfish: A marine jawless vertebrate that scavenges or preys on animals and produces copious defensive slime. Hagfishes are one of the two living lineages included in Cyclostomi.
Cyclostome mouth: The circular, jawless oral opening characteristic of hagfishes and lampreys. Its form supplies the clade’s name and replaces the biting jaws of other vertebrates.
Myllokunmingia: An Early Cambrian fossil vertebrate from China, known from exceptionally old deposits. Its fossils provide context for the early diversification of vertebrate lineages near cyclostome origins.
Gnathostome: A vertebrate belonging to the jawed vertebrate lineage, including cartilaginous fishes and tetrapods. Gnathostomes contrast with cyclostomes in possessing jaws and paired appendages.
Jawless vertebrate: A vertebrate that lacks jaws, including living cyclostomes and several extinct lineages. Cyclostomi is the living jawless vertebrate clade, not the full range of jawless vertebrates.
Lamprey: An eel-shaped jawless vertebrate whose adults may attach to other animals with a toothed oral disc. Lampreys are the other living lineage included in Cyclostomi.
Lamprey oral disc: A suction-forming lamprey mouth bearing keratinized teeth around a central tongue. It enables many parasitic lampreys to attach to hosts and rasp their tissues.
Haikouichthys: An Early Cambrian fossil chordate from China often interpreted as a very early vertebrate. It offers a comparison for reconstructing the primitive anatomy of jawless vertebrates.
Chondrichthyes: The vertebrate class comprising sharks, rays, skates, and chimaeras, with skeletons largely made of cartilage. Cartilaginous fishes show that a flexible skeleton does not imply membership in Cyclostomi.

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