Fig. 2. Ultrastructure of the ectodermal muscle in a subumbrellar segment. Both A and B are radial sections, with the radial muscle (RM) cut longitudinally and the circular (swim) muscle (CM) in cross section. Note that the radial muscle cells are superficial to the circular muscle. In both A and B, neurites (N) are present, and contain dense core vesicles. The neurite in A is superficial to the radial muscle, whereas the neurite in B is between the radial and circular muscle processes. In both panels, the subumbrellar cavity is to the top left.