It is well known that many of protozoans equipped with contractile systems by which they enable their bodies to move. A peritrich ciliate, such as Vorticella, Carchesium or Zoothamnium, has a special contractile system in a long stalk projected from the end of the zooid.
The stalk of Carchesium has been studied by both the physiological and morphological methods. Sugi(1961) measured changes of the volume and the length of a Carchesium stalk, and reported that the stalk shortened by 22-33% and the volume decreased by 24-38% of its resting state during contraction. Yagiu and Shigenaka (1960) carried out electron microscopic studies on fibrillar systems in ciliates. They classified Vorticella myonemes, each myoneme being composed of a bundle of fine myofilaments, into 2 types by their distribution in the zooid, body myoneme and circular myoneme. Recently, many papers have been published on peritrich ciliates with electron microscope. Amos (1972) revealed the structures of stalks in Vorticella and Carchesium in relation to a coiling mechanism on the stalk contraction. Allen (1973) suggested that the linking structures of myoneme, endoplasmic reticulum and surface reticulum in the stalk of Vorticella were the evidence of a message transfer from the cell surface to the sites of calcium release to trigger myonemal contraction. From the comparative studies on structures with 4 species of Vorticella, Kawamura (1973) reported that on the myoneme systems of these 4 species, there was a little difference in arrangements of the outer canaliculi of body myoneme each other.
A peritrich ciliate, Zoothamnium, has also a long stalk, but few papers have been published on this species. This paper is dealt with fine structure of the myoneme system in the stalk and zooid of Zoothamnium.