The family Entoniscidae (Isopoda-Epicaridea) is included in the superfamily Bopyrina, which comprises the families Bopyridae, Dajidae and Entoniscidae. The Bopyridae are parasitic in the branchial cavity or on the abdomen of decapods, mainly Macrura and Anomura. The Dajidae occur in the incubatory chamber or on the dorsal surface of mysids and euphausiids, and rarely on Brachyura. The Entoniscidae are found in the visceral cavity of Anomura and Brachyura (an exception is Synalpheion, in a macruran, Coutière 1908), surrounded by a membrane of host origin, and communicating with the branchial cavity of the host by a pore. Together with this internal habitat the family has extreme morphological specialization. Females of the Bopyridae have distinct thoracic segmentation and well-developed peraeopods; in the Dajidae the peraeopods are distinct, but segmentation is less evident; in adult females of the Entoniscidae, however, the peraeopods consist of only the enlarged oostegites, with rudimentary endopodites in some species, and the thorax has only traces of segmentation.