The symbiotic polychaete, Eteonopsis geryonicola Esmark (Dorvilleidae, Polychaeta), occurs in the branchial chambers of the crabs Geryon tridens Kröyer, Geryon quinquedens Smith, and Cancer borealis Stimpson in the North Atlantic. The latter is a new host record. The species is herein redescribed and the original genus, Eteonopsis, revived. A historical summary is provided to clarify confusion concerning the original description. The family, lphitimidae, a similar group of branchiate polychaetes, are synonymized with Dorvilleidae. Eteonopsis occurs with significantly greater frequency and intensity in male than female crabs. Gut analyses of the worms failed to discern their mode of nutrition. A key to the Dorvilleidae is provided.