As we all know, Bilateral symmetry, three-layer embryo, metamerism and having a relatively developed head are the common characteristics of the Arthropoda and Annelida, So it is believed that arthropod originated from annelid.

• 由于节肢动物具备环节动物两侧对称、三胚层、身体分节、头部较发达等特征，所以一般认为节肢动物起源于环节动物或类似环节动物的祖先。