Schematic representation of myogenesis in Aeolidiella stephanieae. Anterior faces upwards and the total size of the specimens is approximately 170 μm in A and B, 120 μm in C, 200 μm in D, 600 μm in E, and 1400 μm in F. "a-p" indicates the anterior-posterior axis of the animals. All aspects are in dorsal view except B, C, and D which are in lateral view. Location of the apical tuft and the velar cilia are indicated (purple) as well as the mouth opening (rosy). (A) Early veliger stage larva (5% of development) showing first fibres of the larval retractor muscle (cyan), the accessory retractor muscle (yellow) and the velar ring muscles (red); v marks the velum, s the shell, and cap the capsule. (B) Veliger stage larva (10% of development) with well developed accessory, larval, pedal (blue), and a metapodial (green) retractor muscle; o marks the operculum. (C) Into the shell retracted metamorphic larva (25% of development) showing degenerating velar lobes and muscles (open triangles); e marks the eye. (D) Closely after metamorphosis (30% of development) the body wall musculature comprises longitudinal (light green), circular (black), and oblique (white) muscle fibres. Note the anlage of the buccal musculature (brown) and that the larval retractor muscle is still degenerating. (E) Juvenile Aeolidiella stephanieae (40% of development) with the anlage of oral tentacles (ot), rhinophores (r), and the first cerata pairs (c). (F) Late juvenile A. stephanieae (65% of development) showing the meshwork of outer circular, intermediate oblique, and inner longitudinal body wall muscle fibres as well as growing tentacles and the cerata with cnidosacs (cn); ft marks foot tentacle, a tentacle-like elongation of the propodium.