Fig. 3. Ultrastructure of the ectodermal muscle in a subumbrellar segment. In A, the circular muscle (CM) is cut longitudinally to show its striated nature. It also abuts the mesoglea (M). The radial muscle (RM) is cut in cross section, and shows a lack of regular organization of thick and thin filaments. In B, the end-to-end junction of two circular muscle cells show a desmosome (D) adjacent to a gap junction (GJ).