Figure 9

Development of the proboscis in Micrura alaskensis. DIC images. A--B. A pilidium larva viewed from the apical plate (ap), anterior lobe to the left, stomach (st) to the right. C-H. A developmental series showing lateral views of different larvae, apical plate up, anterior lobe to the left. A. Proboscis rudiment (arrowhead) first appears as a small and discrete cluster of cells in the plane of bilateral symmetry of the larva between the two cephalic discs (cd). B. Same larva as on A, a different focal plane, that passes through the paired cerebral organ discs (cod) and trunk discs (td). C. Similar developmental stage as on A: the proboscis rudiment (arrowhead) is separate from the cephalic discs. D. Subsequent developmental stage at which the proboscis rudiment (arrowhead) fuses with the cephalic discs. E. The proboscis rudiment (arrowhead) grows out of the head rudiment as a distinctly bilayered bud. D. The proboscis bud (arrowhead) reaches the upper margin of head rudiment (hr). G. The proboscis (pb) continues to elongate along the larval esophagus (es) and reaches beyond the margin of the juvenile head (jh) at extended proboscis stage. H. The distal tip of proboscis (arrowhead) reaches the stomach (dark magenta shape) at complete proboscis stage.