The opercular bond in the egg-shell of the monogenean Entobdella soleae, aplatyhelminth skin parasite of the common sole (Solea solea)

The monogenean (platyhelminth) skin parasite Entobdella soleae from the com mon sole (Solea solea) lays tetrahedral eggs. One of the 4 corners of the t etrahedron 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 o f about 2 mu m), is more or less uniform in thickness (30-38 nm), or tapers slightly. About 345 nm from the outer surface the cement layer narrows abr uptly to about 10 nm. The cement is exposed on the inner surface of the she ll, but in most eggs a la?;er 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 th e narrow outer region initially remained intact. These observations are dis cussed in relation to the following (1) survival of the eggs during embryon ic development, (2) hatching, (3) the 'hinge' often connecting the operculu m to the empty egg-shell, (4) the rapid hatching that occurs in some other monogeneans.