ミズクラゲ,Aurelia aurita,エフィラの横紋筋の微細構造

Coelenterates generally have smooth muscles in bodies (Hyman, 1940 ; Kawaguti, 1966, Kawaguti and Yoshimoto, 1973, Matsuno, 198lb), excepting some peculiar occasions (Chapman, 1978). But in the free swimming medusae and ephyrae, cross striated muscles are observed, although they are simple and undeveloped in their structures as compared with those of mammals and arthropods. That is, they are characterized to have short sarcomeres, short A and I bands (Kawaguti and Hamakoshi, 1963 ; Keough and Summers, 1976) and thin myofibrils. And sometimes they lack the sarcoplasmic reticular systems (S. R) (Matsuno, 198la).
Free swimming medusae develop through the developmental course of ephyrae which were released from the strobila. So it must be reasonable to assume that cross striated muscles in medusae develop through the developmental course of the ephyra. Aurelia aurita is a convenient material to examine such development, that is, the polyps are easily transformed into strobilae under a temperature treatment in the laboratory (Kakinuma, 1962).
Cross striated muscles in an Aurelia ephyra were observed in a circular muscle around the mouth and radial muscles running a longitudinal direction in the lappets. These two muscles contain a similar type of cross striated muscle cells which resemble those of the medusae, but those two muscle cells were different from the fine structures.
This paper deals with the fine structures of both cross striated muscle cells in comparison with those of other medusae and ephyrae, and discusses the relation of the circular and radial muscular systems.