The body of bilateral symmetrical animals is developed from three layers of germ layers, that is, it has mesoderm. Three germ layers evolved from two germ layers. Therefore, bilateral symmetric animals are superior to radiation symmetric animals.

As the name implies, their shapes are symmetrical on both sides. The brain began to form in the front of the trunk. Some kinds of soft tissue are annular muscle (contraction will lead to narrowing of trunk) and longitudinal muscle (contraction will lead to recovery of trunk); Most of them have a long intestine and blood system. There is a body cavity between the epidermis and the intestine, which will be filled with body fluids to form hydrostatic pressure to maintain its shape. If there is a blood system, the excretion of animals will be completed through the original kidney and the posterior kidney.

They divide in three ways: spiral division (primordial division), radial division (posterior division) and bilateral symmetrical division (primordial division and posterior division).