Quarterly Journal of Microscopical Science, Vol s3-98, 265-278, Copyright © 1957 by Company of Biologists

The Structure and Hydromechanics of the Musculo-epithelium in Metridium

¹ Zoological Laboratory, Cambridge

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 propsed 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 realaxation. It may be moved passively and has considerable elasticity. Modifications of the musculo-epithelium in certain anatomical regions are discussed.

Although true musculo-epithelium characterized only the coelenterates, analogous systems occur in the tissues of several higher animals, and it is suggested that inter-cellular 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 mesoglocal fluid together form an ‘internal medium’ which may provide some degree of biochemical co-ordination in this animal.