Figure 8

Cerebral organ disc invaginations in a 17-day-old pilidium of Micrura alaskensis. A, A', C, C'. Confocal images of two different pilidium larvae stained with phalloidin (white) and hoechst (orange). A. A z-projection of the entire larva; side view. Apical organ (ap) is at upper right, anterior lobe to the left, stomach (st) to the right. The left cephalic disc (cd), the left cerebral organ disc (arrowhead), and the left trunk disc (td) are discernible as dense clusters of nuclei. A'. A sub-stack (6 μm thick) of the same larva, that highlights the left cerebral organ disc (arrowhead), which originates as an invagination of subumbrellar pilidial epidermis, on the inner side of the lateral lappet (lp) in close proximity to the ciliated ridge (cr). The section also passes through the left trunk disc, the left cephalic disc, the proboscis rudiment (pb), which at this stage already consists of multiple cells, and the stomach. B. A diagram of the pilidium larva viewed in frontal aspect (apical organ up, anterior lobe facing us) that illustrates the plane of sectioning (orange line) for A. Cephalic discs and the proboscis rudiment are omitted for clarity. C. A z-projection of another larva, a slightly oblique view of the space between the lateral lappets and the subumbrellar surface of the posterior lobe (pl). The paired cerebral organ discs (arrowheads) and the trunk discs are discernible as dense clusters of nuclei. C'. A sub-stack (8 μm thick) of the same larva, that highlights the invagination of the right cerebral organ disc (arrowhead). The section also passes through the left and right trunk discs, left and right ciliated ridge, and the stomach. D. A diagram of the pilidium larva in lateral aspect illustrating the plane of sectioning (orange line) for C. See also additional files 4 and 5 -- Movie 4 and Movie 5.