Figure 7

Morphological and ultrasrtuctural changes of the larval hood and tentacles at the firsts stages of metamorphosis of Phoronopsis harmeri. (A-C) The anterior portion of the body of live animals. (D, F, G) Semithin sections. (E, H-K) Thin section. (A) First stage of metamorphosis: the hood (pl) remains its integrity. (B) The next (second) stage of metamorphosis: the hood (pl) turns into cellular debris and engulfed. (C) The third stage of metamorphosis: larval tentacles (t) form a “cup” and surround the hood. (D) The second stage of metamorphosis, sagittal section; the oral side is to the right, the anal side is to the left, the apical is at the top. (E) Thick basal lamina (bl) and spacious blastocoel (bc) under degenerated cells of the preoral lobe and the apical organ, which still remains synaptic vesicles (sv). (F) Sagittal section of the protocoel (c1), degenerated hood (pl), and the apical organ (ao). (G) The cross section of the tentacle, which starts to acquire the definitive style via the peeling of the postoral ciliated band (po). (H) The mediofrontal neurite bundle (arrowheads). (I) The medioabfrontal neurite bundles (arrowheads). (J) The lateroabfrontal neurite bundle (arrowheads), which is associated with gland cell (gc). (K) Laterofrontal neurite bundle (arrowheads) is associated with laterofrontal sensory cell (lfc), which undergoes the cell death. Abbreviations: az – abfrontal zone; bm – blood masses; bv – blood vessel; c2 – tentacular coelom; c3 – trunk coelom; es – esophagus; fz – frontal zone; lfz – laterofrontal zone; m – mouth; mc – muscle cell; mi – microvilli; mv – median blood vessel; n – nucleus; nf – nerve fiber; pt – posterior part of the larval body with the telotroch; sd – stomach diverticulum; st – stomach; tn – main nerve ring; tt – telotroch.