Peritrich ciliates, such as Vorticella and Carchesium, have contractile stalks for their movements. Several papers have been dealt with the stalks as a contractile system in protozoa (1, 2, 6). The contractile structure in the stalk has been named as "Spasmoneme". Electron microscopically, it contains a myoneme which is a bundle of many thin filaments, each with a diameter of 4 to 7 nm, and some cell organelles in the cytoplasm (1, 2, 4, 7). A bundle of myoneme in a zooid (zooid myoneme) is more slender than that in a stalk (stalk myoneme), though a myoneme appears to be similar in fine structure each other. Zooid myonemes are converged into a single stalk myoneme at the region of the scopula, thus the stalk myoneme is conrposed of bundles of zooid myonemes. There is an another myoneme system around an oral region, which acts on contraction of the oral part (6). Zooid myonemes are always surrounded with endoplasmic reticula (E. R), the form of which, in the genus Vorticella, varies with the species (4). Recent investigations suggested that the E. R in Vorticella are sites of calsium release to trigger myonemal contraction (1). The similar structures of E. R were observed in Zoothamnium (5).
A peritrich ciliate Epistylis bears a resemblance to Carchesium or Zoothamnium in size and form under a light microscope, but the contractile movements are somewhat different among them. The stalk in Carchesium contracts into a coil and that of Zoothamnium meanders by the myonemal contraction in the stalk, whereas in Epistylis, the contraction and elongation take place only in the cell body. This paper is dealt with structures of the contractile system within the cell body in Epistylis.