All chordates have a dorsal hollow nerve tube, a notochord, and pharyngeal gill slits. All vertebrates (members of a subphylum of chordata) have a backbone (spinal column) and a closed circulatory system.
Members of the class Agnatha are jawless fish. Examples include lampreys and hagfish. Members of the class Chondrichthyes have skeletons made of cartilage, placoid scales, and lack gill covers. Examples include sharks and rays. Members of the class Osteichthyes have skeletons made of bone, cycloid scales, gills covered with opercula, and a swim bladder (for floating). Fish have a two-chambered heart.
In general, amphibians have smooth, moist skin. The are ectothermic (cold blooded) and must return to water to breed. The members of the order Urodela have tails as adults while the members of the order Anura do not. Amphibians have a three-chambered heart. They have two lungs for respiration, but most respiration is accomplished through the skin and lining of the mouth. For excretion, amphibians use two kidneys.
Reptiles have bodies covered with scales and dry skin. They are also ectothermic. Unlike amphibians, however, reptiles are completely independent of water. The order Rhyncocephalia contains the most ancient species of reptile. In the order Chelonia the teeth are fused to form a beak and the body is enclosed in two shells. Members of the order Crocodilia have thick, leathery skin, large teeth, and a four-chambered heart. However, most other reptiles have three-chambered hearts. The order Squamata contains a two suborders: Sauria (lizards) and Serpentes (snakes).
Birds (class Aves)
Birds are characterized by feathers, wings, beaks, and hollow bones (for flight). They are classified according to living style, foot structure, and beak structure. They have a double circulatory system (veins and arteries) and a four-chambered heart. Both their excretory and digestive wastes are mixed and excreted together through the cloaca.
Mammals (class Mammalia)
Mammals are characterized by mammary glands (a gland that secretes milk for the young), extended parental care, body hair, and giving birth to live young. Monotremes include platypuses and echidnas. They have no true teeth and their legs are out to the side like reptiles. Marsupials include kangaroos and opossums. Their young finishes development in a pouch. Placentals comprise the largest group of mammals. They have an uterus for developing embryos and a placenta for nourishment of the young.