cyclostomes

Cyclostomes are a small, ancient group of jawless vertebrates that includes two living lineages: the lampreys (Petromyzontiformes) and the hagfishes (Myxini). The term cyclostome means “round mouth,” a reference to their lack of true jaws. They are among the most primitive vertebrates still in existence, retaining a cartilaginous skeleton, a persistent notochord, and a skull surrounding the brain. Their bodies are typically eel-like, and they lack true paired fins.

Anatomy and feeding adaptations vary between the two lineages. Lampreys have circular, suction-based mouths with keratinous tooth-like elements used for attaching to hosts or rasping tissue, and they usually have seven pairs of gill openings along the body. Hagfishes possess a toothed, tongue-like structure and barbels around a lipless mouth; they are mostly deep-sea scavengers and can exude slime as a defense. Both groups lack jaws and true vertebral columns, though they have a cranium and cartilage-supported internal structures.

Ecology and life history: Cyclostomes inhabit marine and freshwater environments. Lampreys can be anadromous, living in the sea as adults and returning to rivers to reproduce; many species have a larval stage called ammocoetes, living in sediment for several years before metamorphosis. Some lamprey species are parasitic as adults. Hagfishes are generally detritivores and scavengers of the benthos. Reproduction is sexual in both groups, but lamprey life cycles are well documented, while hagfish reproductive biology is less well understood.

Evolution and significance: Cyclostomes are an early branch of vertebrate evolution and provide key insights into vertebrate origins. Their primitive features contrast with gnathostomes (jawed vertebrates) and help illuminate the evolution of key vertebrate traits. Conservation and human interactions vary; some species face habitat pressures, while the sea lamprey has become an invasive pest in certain freshwater systems.