Fig. 6. Closure of a field of porocytes in the apical pinacoderm correlates with contraction of the choanosome after stimulus by agitation. (A–C) Stereo microscope images show that individual ostia (arrows) close within 50 s, and a field of 20 porocytes closes over a period of 83 s. Scale bar, 100 µm. (D) Constriction (closing) of eight ostia in the field shown in A is correlated with the contraction of the choanosome (broken line). Shortly after the sponge was stimulated the ostia closed and the choanosome contracted. A few ostia opened briefly at 1200 s during an expansion of the choanosome, but these closed again as the choanosome contracted. The field opened again with the expansion of the choanosome at t=1500 s (here approximately 22 min later), and again closed just prior to contraction of the choanosome at 2400 s.