Animal kingdom of the highest animal, is also one of the most successful development of a class. The common feature is that there are three major features of the chordate, dorsal canal and gill cleft in the individual developmental process or at a certain time, including the caudate animal, the cephalic animal and the vertebrate. In addition to the above main features, chordates also have some secondary features: closed circulatory system (except for the tail animal), the heart, if present, is always located in the ventral surface of the digestive tract; the posterior end of the anus, Exist in a certain stage of life or lifetime existence; with the formation of the endoskeletal bones. Invertebrates also have the rear mouth, bilateral symmetry, three germ layers, body cavity and segmentation etc. characteristics.
