Figure 3

Onset of feeding, and early juvenile development in Micrura alaskensis. DIC images. Lateral views. Apical plate up, juvenile anterior left. A. Four-day-old feeding pilidium features an apical plate (ap), extensive blastocoel (bl), esophagus (es) with a pair of ciliated ridges (cr), stomach (st), and circumoral ciliated band (dotted line) spanning the lappets (lp). B. The cephalic imaginal discs (cd) appear in vicinity of the ciliated band at the anterior margin of the lateral lappets after about one week of development. Stomach is colored with pigment derived from algal cells. C. The second pair of imaginal discs, called trunk discs (td), appears in vicinity of the ciliated band at the posterior margin of the lateral lappets as early as 9 days of development. At this stage the anterior larval lobe (al) and posterior lobe (pl) are well developed. D-E. The third pair of imaginal discs, the cerebral organ discs (cod), and the unpaired proboscis rudiment (pb) appear nearly simultaneously, as early as 15-17 days of development. The cerebral organ discs invaginate from the inner surface of the lateral lappets near the esophageal ciliated ridges (cr). F. The unpaired dorsal rudiment (dr) appears between the stomach and the pilidial epidermis in diametrically opposite position to the proboscis rudiment as early as 24 days of development. Note the pilidial posterior cirrus (pc). G. Head and trunk stage. The cephalic discs fuse with each other and the proboscis rudiment to form the head rudiment (hr). The cerebral organ discs together with the trunk discs and the dorsal rudiment form the trunk rudiment (tr). H. Torus stage is characterized by a doughnut of juvenile tissue (juv) around the larval gut formed by the fused head and trunk rudiments. I. Extended proboscis stage is characterized by the proboscis rudiment growing out of the forming juvenile head along the larval esophagus and reaching beyond the head margin.