Hagfish are bottom scavengers. Probably the oldest jawed fish. Arose in Devonian period, placoid scales, great sense of smell, lateral line system, ventrally positioned mouth. They possess light modern cycloid or ctenoid scales. Changes present for adaptation to terrestrial life.
Land has many habitats, more oxygen than water, but is less dense for supporting body than water.
The temperature fluctuates more on land. Minute dermal scales, ray-fin tail, skull similar to Crossopterygian, sensory structures for aquatic environment. Gave rise to Reptiles. Origin may be monophyleti c from the Labyrinthodont, or diphyletic with Anurans arising from Labyrinthodonts and Urodeles and Apodans separately from Crossopterygians. Features include most glandular skin, not scales, keratinized skin, double circuit circulatory pattern.