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Myocytes are long, fusiform cells found in the osculum and other contractile areas of many sponges. Myocytes i n the oscular sphincter of Tedunia ignis and the osculum and dermal membrane of Microciona prolifera were studied with light-and electron-microscopes to compare their structure to that of muscles. Salient points of similarity between myocytes and smooth muscles were their long, fusiform shape, their red color after staining with Mallory's triple stain, and the presence of filaments running longitudinally in the cytoplasm.Microciona myocytes have thick filaments of 150-250 A diameter and thin filament of 50-70 A diameter, and in transverse sections the thin filaments occasionally appear as a ring of dots around a thick filament. Longitudinal sections of Tedania myocytes show only one type of filament, which varies from 100 A to 200-300 A diameter in thick regions of the filament. Although transverse sections show light material around the dense filaments, a distinct pattern of thick and thin filaments is not seen in Tedania.Due to infrequent contacts between cells, the large extra-cellular space observed with the electron microscope (49% in Tedania, 57% in Microciona), and the absence of nerves, each myocyte probably acts as an independent contractile unit. Public Health Service training grant 5TI-GM-619 andNational Science Foundation grant 6-8795. This paper is based on a th'esis submitted to the Graduate College of the University of Illinois in partial fulfillment of the reauirements for the derrree of Doctor of Phil-

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