Fig. 2

Ostiactis pearseae external and internal anatomy, including cnidae. a, b External anatomy of Ostiactis pearseae from Pescadero Basin; a lateral view; b oral view. c Detail of the distal row of papillae in the column (arrows). d Detail of longitudinal section through perforated papillae (cinclide). e Longitudinal section of the distal column showing mesogleal marginal sphincter muscle (area within the rectangle). f Cross section of a tentacle showing ectodermal longitudinal muscles (arrows). g Cross section at the actinopharynx level showing cycles of mesenteries; numbers between mesenteries indicate different cycles. h Detail of marginal sphincter muscle fibers in the mesoglea. j Detail of developing oocytes and lipid inclusions (red small dots) in the gastrovascular cavity. i Detail of spermatic cysts (arrow points to largest cyst). k Cnidae types of O. pearseae: basitrichs (a, c, e, h, k), holotrichs (b, f), robust spirocysts (d, g), p-mastigophores A (i, l), p-mastigophores B1 (j, m). Abbreviations: ep, epidermis; ga, gastrodermis; me, mesoglea; pap, papillae. Scale bars: a–c, 6 mm; d, g, 1 mm; e, 0.5 mm; f, h, i, j, 0.1 mm; k, 25 μm