The Ultrastructural Basis for the Electrical Coordination Between Epithelia of Hydra

Abstract

Previously published electrophysiological and ultrastructural studies suggest that the coordinated behavior of Hydra depends on both nervous and non-nervous conduction (c. f. Josephson and Macklin, 1967; Kass-Simon, 1972; Wood, 1961; Hand and Gobel, 1972). As discussed in another paper in this volume (Kass-Simon, 1976), hydra’s behavioral repertoire seems to require the controlled passage of information between the two muscular cell layers, the ectoderm and the endoderm, which are separated from each other by a thick acellular layer, the mesoglea. To help reveal a morphological basis for this interaction, we undertook a light and electron microscopic study of Hydra, directed exclusively toward defining how the two cell layers interact morphologically. Three separate but related topics were addressed: 1) direct interaction of muscular cells of the two epithelia via gap junctions; 2) bridging of the mesoglea by nerve cell processes, and 3) a third type of cellular process which has been reported to occur within the mesoglea of Hydra, but which is actually a bacterial symbiont.