The monogenean (platyhelminth) skin parasite Entobdella soleae from the common sole (Solea solea) lays tetrahedral eggs. One of the 4 corners of the tetrahedron is a detachable operculum which is bonded to the rest of the egg-shell by cement. Most of this cement layer, beginning at the inner surface of the shell and running through almost to the outer surface (a distance of about 2 μm), is more or less uniform in thickness (30–38 nm), or tapers slightly. About 345 nm from the outer surface the cement layer narrows abruptly to about 10 nm. The cement is exposed on the inner surface of the shell, but in most eggs a layer of shell about 10 nm thick covers the narrow outer region of the cement layer. When experimentally perforated eggs were incubated with trypsin, the wide inner layer of cement was digested, but the narrow outer region initially remained intact. These observations are discussed in relation to the following (1) survival of the eggs during embryonic development, (2) hatching, (3) the ‘hinge’ often connecting the operculum to the empty egg-shell, (4) the rapid hatching that occurs in some other monogeneans.