1. In the present paper it is demonstrated that Actinotrichid mites and Palpigradi are the only Chelicerata in which coxae are absent. Species of these groups present moreover epimeres, prosomatic supracoxal setae, and (primitively) two femora. Both groups are classified here as Chelicerata Acoxata. 2. The evolution of the coxa apparently started with the development of a coxal region. In many groups of Arachnidea the free coxae have extended in ventro-sagittal direction, and secondarily lost their movability. 3. Four groups of Chelicerata are now distinguished: Merostomata, Scorpionidea, Arachnidea s. str., and Chelicerata Acoxata. Two trochanters appear to be present in species of three groups of Arachnidea; the presence of a second trochanter is considered here an ancestral character. Acarologia, t. XIX, fasc. 1, 1977. 2. 4. The evolution of the Chelicerata Acoxata could have passed from animais living in the ocean floor, by way of interstitial animais, to terrestrial soil animais.