Practice question

Class Agnatha includes extant jawless craniates, representing earliest diverging vertebrate lineage lacking jaws derived from mandibular branchial arch, also lacking true paired appendages, dentine and bone in many forms. Characteristics include persistent notochord surrounded by arcualia, single median nostril in lampreys, multiple gill pouches opening separately, keratinized tooth-like structures and cartilaginous skeleton. Modern examples are Petromyzon lamprey and Myxine hagfish. Chondrichthyes, Osteichthyes and extinct Placodermi are jawed gnathostomes. Agnatha thus serves as stem group crucial for understanding evolution of jaws, paired fins and vertebral mineralization in chordate phylogeny.