The subphylum Vertebrata is divided into two divisions on the basis of jaws. Agnatha lack jaws and are represented by the class Cyclostomata. Gnathostomata bear jaws and have two super classes. Super class Pisces includes animals that bear fins, with the classes Chondrichthyes and Osteichthyes. Super class Tetrapoda includes animals that bear limbs, with the classes Amphibia, Reptilia, Aves and Mammalia. Thus the vertebrates are arranged into six classes in this chapter. Within these classes, the heart shows a clear progression. Fishes have a two-chambered heart with one auricle and one ventricle. Amphibians have a three-chambered heart with two auricles and one ventricle, and reptiles usually have a three-chambered heart, although crocodiles have a four-chambered one. Birds and mammals have a completely four-chambered heart. Body temperature regulation also divides the classes. Fishes, amphibians and reptiles are poikilothermous (cold-blooded) and cannot regulate their body temperature, whereas birds and mammals are homoiothermous (warm-blooded) and maintain a constant body temperature. Fertilisation is external in bony fishes and amphibians but internal in cartilaginous fishes, reptiles, birds and mammals. The summary table below, which compares these features across the classes, is a very useful tool for answering NEET questions quickly and accurately.