Figure 2

Schematics of the fate of larval structures during the metamorphosis of Bugula neritina. Descriptions and slightly modified drawings after [21, 22, 76]. A) Competent larva. B) Five seconds (s): post settlement: Initial attachment through eversion of the internal sac. C) 20 s: The roof region of the internal sac moves towards the substratum, the apical disc retracts. D) 60 s: The apical disc re-extends, the corona starts involuting, the pellicle (excreted by the neck region) covers the larva. E) Approximately 120 s: The corona involutes, the pallial epithelium covers the apical hemisphere, the wall rotates. F) Approximately 160 s: The edge of the pallial epithelium constricts and the apical disc is compressed, bringing the pallial epithelium into contact with neck and wall of the internal sac. G) Approximately 240 s. The pallial epithelium constricts and the wall rises towards the apical region. H) 360 to 390 s. The pallial epithelium thickens, the wall covers the apical region, the apical disc and surrounding pallial epithelium are pulled inside. Coronal cells begin to autolyse. I) Several hours: transformation from the preancestrula to ancestrula: the pre-ancestrula elongates, the wall region forms the epidermis (which then secretes the cuticle and calcium carbonate). Invaginated cells of the epidermal blastema form the digestive tract and the lophophore. The splanchnic lining is formed by cells of the mesodermal blastema. The pallial epithelium has formed the tentacle sheath. co, corona (light grey); eb, epidermal blastema (purple); gl, gland cells of pyriform organ; lo, developing lophophore; mb, mesodermal blastema (cyan); nn, nerve nodule; pe, pallial epithelium; pha, pharynx; pel, pellicle, secreted by the neck region; st, stomach; ts, tentacle sheath; internal sac (is) consisting of n, neck (blue); r, roof (dark grey); and w, wall (grey).