Structure of the mouth of Hydra spp. A breach in the epithelium that disappears when it closes

Summary When the mouth of a hydra is not open, the ectodermal and endodermal epithelia are each continuous over the oral surface. All cells are joined to all neighboring cells by septate junctions. A mouth forms when muscles stretch the center of the oral region and pull several cells into a thin lamella. Finally, cell attachments break. Rapidly (within one second) this breach expands as a round opening until it is approximately half the diameter of the surrounding column. Considerable rearrangment of the central cells, and therefore of their septate junctions, likely takes place rapidly as the mouth expands. A few cells may be lost from the epithelia. Closure of the mouth is slow and results in irregular thickening and wrinkling of the tissue. Final closure to restore histological continuity resembles wound healing more than occlusion typical of the mouths of other animals. The lack of a permanent mouth may be a primitive character or an adaptation to a simple body plan, and is shared by many hydrozoan polyps but not by more complex cnidarians.