A New Type of Muscle Cell from the Subumbrella of Obelia

Abstract

The subumbrella of the hydrozoan medusa Obelia consists of three layers of cells. The innermost layer is the endodermal lamella whose broad polygonal cells are in close contact with the middle layer of polygonal myocytes which have striated myofibrils running in two layers at right angles to one another in the same cell. These myofibrils extend beyond the myocytes as plasma membrane-enclosed prongs to make contact with the prongs of neighbouring cells in such a way that whole mounts give the impression that the slender myofibrils are continuous over the subumbrella. Cross-layered myocytes can be seen to be another means of decreasing the surface area of the subumbrella, a condition necessary for medusoid locomotion. Helically banded radial myofibrils are found in the outer layer.