1, Protozoa: Ovipods such as platypus and echidna, which are the most primitive among mammals.
2. Post-mammals: Although this kind of animals are more evolved than primitive mammals, they also belong to an ancient and low class. For example, marsupials, although viviparous, have no placenta, and their cubs develop and grow in their mother's pouch.
3. True mammals: They are the tallest living mammals and the most evolved group in vertebrates and even the whole animal kingdom. According to statistics, there are more than 450 species of mammals in China, accounting for 10.6% of the total number of mammals in the world, and there are 50 families of * * orders 14 orders. Their main characteristics are as follows: there is a spine connected by many vertebrae in the body, with hair on the body surface, viviparous, breastfeeding, constant temperature, developed brain, and complete separation of the left and right ventricles of the heart.