cyclostomata

Cyclostomata, or cyclostomes, is a group of living jawless fishes that includes hagfishes (Myxini) and lampreys (Petromyzontiformes). In many classifications they are treated as the surviving jawless vertebrates within the broader context of the Agnatha, contrasted with the jawed vertebrates (Gnathostomata).

Anatomically, cyclostomes lack true jaws and paired fins. Their skeleton is cartilaginous, and their mouths are circular, bearing keratinized oral plates rather than teeth. They possess a cranium and a notochord; lampreys have a more developed skull and a primitive vertebral element, while hagfish have a simpler axial skeleton. They also feature a lateral line system, a single nostril, and gill openings along the body.

Ecology and life history: most species are aquatic. Lampreys include both parasitic and non-parasitic forms; ammocoete larvae are filter feeders living in sediment before metamorphosis into adults. Hagfish are scavengers, notable for slime production as a defense. Reproduction differs between groups: lampreys undergo external fertilization, with eggs hatching into free-swimming ammocoetes; hagfish reproduction is less well understood and generally does not involve a distinct larval stage.

Evolution and classification: today the group comprises two living orders, Myxini and Petromyzontiformes. Cyclostomes are among the most primitive living vertebrates, and their study informs vertebrate evolution, particularly the transition from jawless to jawed forms.