Sketch drawings highlighting the development of individual ganglia into a centralized brain in I. notoides . In stages older than stage 19, the neuropil (black) consolidates in the center of various brain lobes. (A) Dorsal view of a stage 19 embryo (anterior faces up). (B) Ventral view of a stage 19 embryo (anterior faces up). (C) Posterior view of a stage 19 embryo (dorsal faces up). (D) Sagittal section along the midline of a stage 23 embryo (anterior faces to the left). (E) Sagittal section along the midline of the anterior body regions of a stage 30 hatchling (anterior faces to the left). The esophagus is marked by a thick dashed line and the internal and external yolk portions are outlined by a thinner dashed line in D and E. The statocysts are marked by asterisks. Abbreviations: a, arm; e, eye; f, funnel; m, mantle; y, yolk. Modified from [8, 12]. Scale bars 150 μm.