SCANNING ELECTRON-MICROSCOPY OF THE MUSCLE SYSTEM OF HYDRA-MAGNIPAPILLATA

The fine structure of the ectodermal and endodermal muscle layers of H ydra magnipapillata has been analyzed by scanning electron microscopy after hydrolytic removal of the mesoglea with NaOH and subsequent expo sure of the basal and lateral aspects of the layers by mechanical diss ection. The ectodermal muscle layer consists of fibrous processes of e pithelial cells extending longitudinally to the body axis, whereas the endodermal muscle layer comprises cells with hexagonal bases and seve ral strands of myonemes oriented circularly. In each layer, the muscul ar elements tightly interdigitate, extending a continuous muscle sheet along the mesoglea. The ectodermal and endodermal muscle sheets commu nicate with each other via foliate microprojections penetrating the me soglea. On the lateral aspect of the ectodermal epithelium, spiny nerv e fibers run along the upper surface of the muscle processes. The spin es are often attached to muscle processes, suggesting that the former monitor muscle contraction. Nerve fibers occasionally come into contac t with the mesoglea through narrow gaps between the muscle processes. In the hypostomal ectoderm, a small spindle-shaped cell, probably sens ory in nature, extends an apical cilium and a long basal process.