At present, there are about 1.5 million species of animals in the world, which can be divided into two categories: unicellular protozoa and multicellular metazoa. The species of metazoa are dozens of times more than protozoa, which can be divided into two categories: one is that the cells of animals only have different differentiation, such as porous animals (sponges); The cells of another animal have a division of labor and can form a completely fixed system of tissues and organs. Multicellular animals except porous animals all belong to this category. In addition, most zoologists are used to dividing the whole animal kingdom into invertebrates and vertebrates according to whether there are vertebrae in animals. This division is not very scientific. To be exact, animals should be divided into two categories: chordates and chordates.

There are more than 6.5438+0.5 million species of animals, most of which belong to invertebrates, while vertebrates account for only a small part, belonging to a subfamily of Chordata. Zoologists tend to classify invertebrates into 3 1 phylum in recent years. Some people abroad include Placozoa established by Professor Grell of West Germany 197 1 and Loricifera established by Kristensen of Denmark in recent years, which divides invertebrates into 33 phyla. Although there are great differences in animal species, it is generally believed that they are all descendants of the same ancestor, have a certain kinship, and are the products of long historical development.