In this work, we provided a genomic perspective of the evolution of classical cadherins among bilaterians, with a focus on the arthropods, including spider and isopod. Prior to this work, we hypothesized that the two distindt forms of vertebrate and insect classical cadherins, type I and type IV, which are popular in cell-cell adhesion studies, independently evolved from a larger ancestral cadherin through reductive changes. Extensive genomic analyses presented supported this hypothesis.