The ecology and feeding biology of two shell-burrowing polydorids associated with hermit crabs from the Philippines were investigated. Polydora umangivora and P. robi were found in shells of 15 and 28 gastropod species, respectively, occupied by nine hermit crab species. Polydora umangivora constructs U-shaped burrows in the outer shell while P. robi constructs burrows in the apex of the shell, extending to an opening along the columella of the upper body whorls. Polydora umangivora and P. robi preyed upon the embryos of three and seven species, respectively, of diogenid hermit crabs collected in the field. In laboratory feeding trials, P. robi ingested up to 230 developing eggs and 70 embryos of Calcinus gaimardii over a 6-h period. The impact of embryo predation is more apparent in hermit crab species that produce small broods of eggs (Paguristes runyanae) than in species that produce large broods of eggs (Calcinus gaimardii). Polydora robi transports the embryos to the mouth by a combination of muscular and ciliary action of the palps; ingestion follows breakage of the attachment stalk between embryo and pleopod seta. Invertebrates known and suspected to feed on host hermit crab eggs are reviewed and the evolution of polydorid/hermit crab symbioses is discussed.