Myxini

Myxini, commonly known as hagfishes, are a small, early-diverging group of jawless fishes that belong to the cyclostomes, together with lampreys. They inhabit marine environments worldwide and are characterized by an eel-like body, a cartilaginous skull, and the absence of a true vertebral column. They retain a notochord as a primary axial support and lack paired fins or jaws.

Anatomy and physiology: Hagfishes have scaleless, slimy skin and a single median nostril. They possess multiple gill openings along the body and a mouth equipped with keratinized dental plates on a rasping tongue. The body is adapted for a scavenging lifestyle, and unique slime glands along the sides can produce copious mucus as a defense.

Ecology and behavior: They are bottom-dwelling and occupy depths from shallow coastal waters to the deep sea. Hagfishes feed on carrion and invertebrates, and can bore into the bodies of dead animals to feed internally. A distinctive knotting behavior helps them pull themselves through carcasses and aid in feeding.

Reproduction and life history: Reproductive biology in hagfishes is not well understood. Observations are sparse, and details of mating, fertilization, and development remain largely unknown.

Evolutionary significance: As one of the most primitive surviving vertebrate lineages, hagfishes provide important clues to early vertebrate evolution, particularly the origin of vertebrates prior to the evolution of jaws. The fossil record includes hagfish-like forms, and ongoing research continues to refine their exact relationships to other vertebrates.