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.

gill slits are ventrally placed

head and turnk are widened considerably

distinct demarcation between body and tail

their pectoral fins distinctly marked off from cylindrical bodies.

Spiracle in metathorax, trachea, tracheloes, oxygen diffuses into cells

Mouth, bronchial tube, trachea, oxygen enters cells

Spiracles in prothorax, tracheoles, trachea, oxygen diffuses into cells.

Hypopharynx, mouth, pharynx, trachea, tissues