The word Agnatha is formed from the Greek word a meaning jaw and gnathos meaning jaw. Agnatha means jawless. The word Cyclostomata is formed from the Greek word cyclos meaning round and stoma meaning mouth. Vertebrates that lack jaws, scales and jointed appendages are called Cyclostomata. Their endoskeleton is cartilaginous and has 5-15 pairs of gills. They are adapted to a parasitic or carnivorous diet. The living Hagfish, Lamprey and the extinct Ostracoderm) include Agnatha. Agnatha consists of 2 categories. Mixini and Petromyzontida.