Flat animals have three germ layers. Flat animals, generally referred to as the phylum of flat animals, are a phylum of the animal kingdom. Invertebrates are a kind of animals with bilateral symmetry, three germ layers, no body cavity, no respiratory system, no circulatory system, mouth and no anus. Flat animals have three germ layers: ectoderm, endoderm and mesoderm. Flat animals flat animals live in wet places such as fresh water and sea. There are two photosensitive pigment spots at the front of the body. About 15000 species of flat animals have been recorded.
According to the different morphological characteristics and lifestyles of flat animals, this phylum can be divided into the following three classes: planaria, trematode and tapeworm.
Among flat animals, free-living planaria is the most primitive group. Trematode is a class of species evolved from the adaptation of planaria to parasitic life. （BianXingDongWuJiGePeiCeng）