body covered with moist skin which is devoid of scales, the ear is represented by a tympanum, alimentary canal, urinary and reproductive tracts open into a common cloaca
fresh water animals with bony endoskeleton, air-bladder to regulate buoyancy
marine animals with cartilaginous endoskeleton, body covered with placoid scales
body covered with dry and cornified skin, scales over the body are epidermal, they do not have external ears.
