Musculo-epithelial cells have been isolated from mesenteries of the sea-anemone Metridium senile, and the descriptions of earlier workers confirmed. The cells contribute both to the muscle-field above the mesogloea and to its overlying epithelium. In sections or whole mounts it is possible to see numerous vertical strands passing from the epithelial elements to their muscle-fibres. The protoplasmic strands are separate from one another and are thus surrounded by fluid which forms a continuous thin layer between the epithelium and muscle-field. It is proposed to call this the subepithelial fluid. Epithelial elements from contracted mesenteries are much taller than those from stretched tissue. As the area of the mesentery decreases during contraction a reversible change from pavement to columnar epithelium takes place. The epithelium is able to follow rapid contractions without delay, owing to the hydrostatic action of the subepithelial fluid in thrusting it outward. There is as yet no evidence that the epithelial protoplasm moves by its own activity during contraction or relaxation. It may be moved passively and has considerable elasticity. Modifications of the musculo-epithelium in certain anatomical regions are discussed. Although true musculo-epithelium characterizes only the coelenterates, analogous systems occur in the tissues of several higher animals, and it is suggested that intercellular fluid may have a hydrostatic function in these situations also. The possible metabolic role of subepithelial fluid in Metridium is discussed, and it is suggested that it and the mesogloeal fluid together form an 'internal medium ' which may provide some degree of biochemical co-ordination in this animal
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