Morphology of branch buds. a A diagram of sagittal section of the main tentacle is shown. The tentacle is a tube-like structure mainly composed of two layers of epithelial cells: outer epidermal epithelio-muscular cells (EMCs), which have basal myoid processes containing contractile muscle fibrils running longitudinally shown in green (Mp), and inner endoderm cells (En) . These two cell layers are separated by a jelly-like substance called mesoglea (Mg). Other components, such as nematocytes and interstitial cells, are omitted for simplicity. Can: canal, Nuc: nucleus. b, c Diagrams showing how the branch buds were viewed in d and e. The tentacles are oriented with their adaxial sides up. The distal portion of the tentacle, which was cut off when observed, is outlined in broken line in c. ⓪: the main tentacle; ①: the first branch; ②: the branch bud forming the second branch. Mp: myoid processes in the section of the tentacle. d, e Confocal images for the buds of the second branch (Branch②) stained with Phallacidin. Adaxial views. These images are generated from multiple Z-sections processed with maximum intensity projections. The tips of the buds were roughly estimated by the highest points of the EMC layers shown in the XZ (under the image) and YZ (right side of the image) sections and are indicated by the points of intersection between the vertical and horizontal yellow broken lines. The yellow lines also indicate the positions of the XZ and YZ sections. The same two images for the XZ and YZ sections in d are shown with ones outlining the apical and basal sides of the EMC layers (white broken lines). Buds in d and e are at different stages of bud formation. Mp: myoid processes. Um: medusa umbrella. Scale bars: 50 μm.