cyclostome

Cyclostome, or Cyclostomata, is a group of living jawless vertebrates that includes two lineages: hagfishes (Myxini) and lampreys (Petromyzontiformes). The name, meaning round or circular mouth, refers to their circular oral openings. Cyclostomes occupy a basal position in the vertebrate family tree, as the sister group to jawed vertebrates (gnathostomes), and they diverged early in vertebrate evolution.

Morphology and anatomy are distinctive. Cyclostomes are generally elongated and eel-like, lacking true jaws and paired fins. Lampreys have a circular, keratinized oral disc with dental plates used for feeding, while hagfishes have a flexible circular mouth and a set of dental papillae. Their skeleton is cartilaginous, and in lampreys the vertebral column is rudimentary rather than a fully ossified backbone; hagfishes retain a prominent notochord. Gas exchange occurs through gill openings, with lampreys possessing seven external gill slits and hagfishes showing multiple gill openings along the body. Hagfishes are notable for slime glands that secrete copious mucus as a defense.

Ecology and life history differ between the two groups. Lampreys include freshwater residents and anadromous species that migrate between freshwater and the sea; many are parasitic as adults, attaching to other fish to feed. They have a larval stage called ammocoetes that lives in sediments for several years before metamorphosis. Hagfishes are primarily scavengers and predators of small invertebrates and dead fish. They lay relatively little emphasis on parental care, and reproduction in hagfishes is not as well understood as in lampreys.

Evolutionary significance is substantial: studying cyclostomes sheds light on early vertebrate anatomy and the origin of jaws, revealing which features are ancestral and how the vertebrate body plan has evolved since the earliest divergence of vertebrates.