AbstractIn some planulae of the alcyonarianGersemia rubiformisan interior structure was found in the entodermal cavity. It consisted of two layers separated by a basement membrane, the inner layer facing a central cavity. The outer layer was structurally identical with the inner “entomesoderm” of the body wall, while the inner layer of the “ball” was provided with numerous cilia and a brushborder. The development of the interior structure is described as appearing from the structural findings in different planulae. This development is very different from any other developmental process described