Fig. 7. Formation of the adult nervous systems in S. japonicus. (A, C–F) 1F9
immunoreactivity, (B) serotonergic immunoreactivity. (A, B) Pentacutula larva.
The most anterior ciliary ring nerve tract (white arrowheads) shows serotonergic
immunoreactivity, but the nerve ring of the adult nervous system (white double
arrowhead) does not. They are two different structures. Nerve tracts of the
primary (buccal) tentacles branch out from the nerve ring anteriorly (yellow
arrowheads). Radial nerves can be seen. (C) Settled juvenile stage. The ciliary
ring nerve tract has disappeared, and the nerve ring of the adult nervous system
(white double arrowhead) has moved to an anterior position, close to the oral
opening. (D) Settled juvenile with three radial nerves (arrows). The nerve ring is
a square. (E) Settled juvenile with four radial nerves (arrows) and a square nerve
ring. (F) Settled juvenile with a pentagonal nerve ring. Five radial nerves
(arrows) are present. The radial nerve with the yellow arrow in (E) and (F) has a
branching structure (yellow double arrowheads), the nerve tract of the
ambulacral primary podia. Scale bars: 100 212