The Integument of the Marine Echiuran Worm Urechis caupo.
During low tide, the burrow water of the marine echiuran worm Urechis caupo becomes hypoxic, and hydrogen sulfide concentrations reach levels that would be toxic to most animals. Integument morphology in U. caupo is evaluated as an exchange surface and as a permeation barrier. Adaptive features include the rugose nature of the epidermis, which increases the surface area for oxygen uptake, and the thick muscular body wall, which provides a chief motive power in creating peristaltic movements along the body wall to ventilate the burrow. The epidermis is covered by a cuticle and contains two types of mucus-secreting cells: orthochromatic and metachromatic. Underlying connective tissue and three muscle layers form the bulk of the body wall. The integument does not present a significant structural barrier to permeation, although the mucus secreted by the epidermal cells may retard sulfide entry. Ultrastructural studies suggest three possible mechanisms that U. caupo may use to counteract the toxic effects of sulfide at the integumentary surface: metabolism of symbiotic bacteria embedded in the innermost cuticle layer and grouped together in the superficial epidermis, dying off of peripheral, sulfide-exposed cells, and oxidation of sulfide at specialized, ironrich, lysosomal organelles termed sulfide oxidizing bodies.