The external morphology of the adult stage of four species of Anomura (hermit crabs), Upogebia pugettensis (Dana), Paguristes turgidus (Stimpson), Pagurus beringanus (Benedict) and Orthopagurus schmitti (Stevens) is compared and the larval stages described. Emphasis is placed on the value of larval stages in clarifying our understanding of the taxonomy and interrelationships of the group. This study seems especially important in the Anomura because of the modifications of the adults which fit them for so specialized a mode of existence. It seems probable that some of the similarities exhibited by them are due more to convergence, as a result of similar mode of life, than to close affinity. This is illustrated by the superficial similarity of the adults of Paguristes turgidus and Pagurus beringanus and the dissimilarity of their larvae, as contrasted with apparent differences between the adult Pagurus beringanus and Orthopagurus schmitti, whose larvae are alike. By a comparison of the least modified appendages (the mouth parts) of the adults, the same relationship becomes apparent as was suggested by a study of the larvae. It is therefore concluded that by a comparison of both the larval and adult phases a clearer understanding of the affinities of the species under consideration is obtained.