ABSTRACT
The structure of the radial muscle of a sea urchin, Asthenosoma ijimai, was studied by light and electron microscopes to understand the physiological properties of the muscle. The radial muscle is smooth one and the diameter of the cell is approximately 4–10um, though the shape of the cross section of the cell is irregular and different from cell to cell. The muscle bundle contains a large amount of connective tissue relative to the contractile cells; approximately half of the cross sectional area is non-muscle. The muscle cells extend long processes by which neighbouring cells are in contact with each other. The muscle cell has two kinds of filaments: thick and thin, measuring 20–40nm and 5–6nm in diameter respectively.
