The Fine Structure of the Mesenteries of the Sea-Anemone Metridium senile

Sections of the retractor and radial face of mesenteries of Metridium senile (L.) fixed withosmic, osmic-phosphotungstic, or formol-phosphotungstic fixatives were examined by electron microscopy. Each muscle-fibre forms the basal part of a musculo-epithelial cell and is in contact through the cell membrane with a differentiated surface layer of the underlying mesogloea. The muscle-fibres bear crests of cytoplasm containing mitochondria. Each crest is continuous with a cytoplasmic stem which passes through the intercellular space into the epithelial part of the musculo-epithelial cell. The musclefibre consists of densely packed longitudinal filaments 40-80 Å across. The musclefibres present a similar appearance over a great range of extension of the muscle. The filaments show no gross periodic structure like that of striated muscle; nor has any fine regular structure like that described in some other ‘plain’ muscle-fibres yet been detected. There is, however, an irregular beading on the filaments at about 240 Å. Though much smaller, the muscle-fibres of the very slowly contracting radial muscle sheet have the same fine structure as those of the very rapidly contracting retractor muscle. The epithelial part of the cell bears a flagellum arising from a cytoplasmic ‘crater’. The flagellum contains vesicles as well as the typical fibrils. It passes continuously into a complex basal corpuscle which continues into a striated fibrous root. Near the epithelial surface there are conspicuous cross-connexions between adjacent cells. Mucus cells and amoebocytes are described. The mesogloeal lattice of Chapman, which permits free extension of the actinian body, is composed of fibres which seem to be built up of tubular fibrils of about 100 Å diameter. The fibrils show well-developed banding at 260 Å. Where fibrils adhere together their banding is congruent. The criteria of correspondence between the electron micrographs and the living structure are discussed, and the value of information from artifacts and varied methods of fixation is noted.