Crocodiles are reptiles of the Crocodylidae and Alligatoridae families, belonging to 8 genera and 23 species. Crocodiles have a longer torso; Muscular development; Flat head, skin tightly attached to the skull, strong skull connection, unable to move; The teeth are cone-shaped and grow in the alveolar cavity, with more than 25 teeth on each side; The tongue is short and flat, unable to extend outward; Small and slightly protruding eyes; The tail is thick and slightly flattened; Short and thick limbs; The external nostril is located on the back of the snout and has a valve on top; There are scales like armor. As one of the oldest species on Earth to date, it retains many features of early dinosaur reptiles and is also known as "living fossils".