In the crab Carcinus the Y organ (a functional molting gland) and the mandibular organ are distinct. In the lobster Homarus and the crayfish Orconectes no structure obviously comparable to the Y organ of Carcinus can be demonstrated, and in these and other macrurans the terms Y organ, ecdysial gland, and molting gland have been applied to the mandibular organ, which is probably not a molting gland in the classical sense.