Cyclostomata

Cyclostomata are a group of jawless vertebrates comprising two subclasses: Petromyzontiformes (lampreys) and Myxiniformes (hagfishes). Cyclostomes have simple, ancient body plans and represent the earliest stage of vertebrate evolution. They are the only living survivors of a group that existed more than 400 million years ago. These animals have a cartilaginous skeleton, no backbone, and lack paired appendages or jaws. Their mouths are circular and contain a sucker-shaped disc of fleshy teeth. The most widespread species of cyclostomes are lampreys, which can be found in most temperate freshwater habitats. Hagfishes inhabit the ocean and are important components of aquatic food webs. Cyclostomata are used in a range of medical studies, particularly to investigate their regenerative capabilities, and lampreys are used as model systems for developmental biology.