Fig. 6. Summary diagrams of the transition of the nervous system in S. japonicus
larvae as inferred from Figs. 4, 5 and unshown data. Nerve tracts that line the
ciliary bands are shown with the thick lines in all larvae. The first ciliary ring
(cr1 in Figs. 1, 5) nerve tract of the doliolaria is made mainly from the anterior
ventral ridge ciliary band nerve tracts (red). The nerve tract that bridges the right
and left nerve tracts at the anterior ridge of the auricularia (pink) is recruited into
the dorsal portion of this nerve ring. The second anterior ciliary ring (cr2) nerve
tract is formed asymmetrically, from the anterior dorsal ridge ciliary band nerve
tracts (orange) plus the right portion of the anterior ventral ridge ciliary band
nerve tract (yellow). The third anterior ciliary ring (cr3) nerve tract is formed
from the mid-dorsal ridge ciliary band nerve tracts (green). The fourth ciliary
ring (cr4) nerve tract is generated from the posterior ventral (light blue) and the
posterior dorsal (blue) ridge ciliary band nerve tracts. The fifth ciliary ring (cr5)
nerve tract is made by elongation and fusion of the posterior lateral ridge ciliary
band nerve tracts (purple). Nerve tracts that are not reused for ciliary ring nerve
tracts are retained in the doliolaria (thin black). Thus, the ciliary ring nerves are
not completely circumferential and remain connected to each other. The
auricularia posterior ventral ridge and the postoral ciliary band nerve tracts
elongate to the anterior.