On the fine structure of opisthobranch egg masses (Mollusca, Gastropoda)

The light-microscopic structure and ultrastructure of the egg masses of som e species of Opisthobranchia (Mollusca, Gastropoda) are presented and discu ssed. The egg masses have a rather uniform structure. The embryos are embed ded in a viscous mass of albumen in some species of "Cephalaspidea s. l.", all Anaspidea and Sacoglossa, whereas in other taxa examined (some "Cephala spidea s. l.", and Nudibranchia) this albumen is missing in the egg capsule s. Instead, the albumen forms an additional compact albuminous layer. This layer seems to dissolve during intracapsular development and is apparently ingested by the veliger larvae in the capsules. Embryos and albumen, or emb ryos and albuminous layers are enveloped by a mucoid membrane, forming dist inct capsules. The capsules themselves are surrounded by an inner mucous la yer and embedded in a mucous matrix. Along the outside of the egg mass a mu lti-layered outer mucous cover is present. Interspecific differences in thi s general structure are apparent in the number of embryos per capsule, the thickness of the capsule, the fine structure of the capsule and the density and fine structure of the mucous layers.