Cephalization, development of highly specialized sensory structures and a highly specialized brain. Closed circulatory system. It is a closed system. The cells of the body exchange food and gases through tiny blood vessels called capillaries. The kidney is the main excretory organ.
Sexes are separate.
Fertilization may be internal or external depending on the species. Respiratory organs include the gills, lungs, or highly vascularized skin. Oxygen is carried to all parts of the body in a liquid called blood. Hemoglobin is the main attractant of oxygen and is found in the red blood cells. Placoid scales, cartilage skeleton internal fertilization, paired fins, Sharks, Rays, and Skates Osteichthyes Bony skeleton, paired fins, operculum, swim bladder Tuna, Bass, Catfish, Flounder Amphibia Smooth skin, no claws, aquatic larvae, breathe through lungs and skin Frog, Salamander, and Toad Reptilia Amniote egg, rough,dry skin, respiration through lungs Snakes, Lizards, Turtles, and crocodiles Aves Feathers, amniote egg, extensive lung system, and wings Birds Mammalia Hair, diaphragm, mammary glands, and bear live offspring.