Galeommatoidea is a superfamily of bivalves that exhibit remarkably diverse lifestyles. Many members of this group live attached to the body surface or inside the burrows of other marine invertebrates, ranging from crustaceans to sea cucumbers (holothurian), heart urchins (echinoid), sea anemones (cnidarian), peanut worms (sipunculan), and spoon worms (echiuran). These symbiotic species exhibit high host specificity, commensal interaction with hosts, and extreme morphological and behavioral adaptations to symbiotic life. Host specialization to various animal groups has likely played an important role in the evolution and diversification of this bivalve group. However, the evolutionary pathway that led to their ecological diversity is not well understood, in part because of their reduced and/or highly modified morphologies that have confounded traditional taxonomy. This study elucidates the taxonomy of the Galeommatoidea and their evolutionary history of host switching based on a molecular phylogenic analysis of 33 galeommatoidean and five putative galeommatoidean species belonging to 27 genera and three families using the 18S and 28S ribosomal RNA genes.