Light (A, B) and scanning electron (C-I) micrographs of Arcella intermedia. (A, B) View of several specimens to illustrate variability in shape and size of the shell. (C) Latero-apertural view showing apertural invagination. (D) Apertural view to show smooth apertural surface. (E) Dorsal view showing regular depressions on aboral surface of the shell. (F) Lateral view. (G) Close up view of aperture to show the apertural lip. (H, I) Portion of shell surface showing regularity in arrangement of the organic hexagonal alveoli and numerous small pores.