but positive signals for 1E11 were seen in adult nerves of the
primary tentacle (data not shown). As the doliolaria larva
turned into the pentacutula larva, the ring nerve, with nerve
tracts of the primary tentacles branching from it, was still at
the middle portion of the larva, well inside the oral opening
(Fig. 7A). It was a completely different structure from the
most anterior ciliary ring (cr1) nerve tract, obvious from the
difference in their positions and immunoreactivity (Figs. 7A,
B). When the larva settled and the primary tentacles protruded
from the oral opening, the ring nerve had moved to an anterior
position near the oral opening (Figs. 7C–F). Radial nerves
were already present when the nerve ring was still at the midposition
of the doliolaria larva (Fig. 7A). The five radial
nerves did not appear concurrently. Some juveniles possessed
only three or four radial nerves (Figs. 7D, E). The nerve ring
in these larvae was not round but square in shape, with the
radial nerves extending from the corners. The radial nerves
within a single larva had different lengths and thickness
between each other. In juveniles with five radial nerves, the
nerve ring became a pentagonal shape (Fig. 7F). The radial
nerves still showed different degrees of formation, and one of
them showed a branching structure, which is the nerve tract of
the ambulacral primary podia (Figs. 7E, F).
Discussion
We have reported nervous system development in the sea
cucumber S. japonicus. This is the first detailed report of
nervous system development, from bilateral embryos to
pentaradial juveniles, in an echinoderm with the ancestral
but positive signals for 1E11 were seen in adult nerves of theprimary tentacle (data not shown). As the doliolaria larvaturned into the pentacutula larva, the ring nerve, with nervetracts of the primary tentacles branching from it, was still atthe middle portion of the larva, well inside the oral opening(Fig. 7A). It was a completely different structure from themost anterior ciliary ring (cr1) nerve tract, obvious from thedifference in their positions and immunoreactivity (Figs. 7A,B). When the larva settled and the primary tentacles protrudedfrom the oral opening, the ring nerve had moved to an anteriorposition near the oral opening (Figs. 7C–F). Radial nerveswere already present when the nerve ring was still at the midpositionof the doliolaria larva (Fig. 7A). The five radialnerves did not appear concurrently. Some juveniles possessedonly three or four radial nerves (Figs. 7D, E). The nerve ringin these larvae was not round but square in shape, with theradial nerves extending from the corners. The radial nerveswithin a single larva had different lengths and thicknessbetween each other. In juveniles with five radial nerves, thenerve ring became a pentagonal shape (Fig. 7F). The radialnerves still showed different degrees of formation, and one ofthem showed a branching structure, which is the nerve tract ofthe ambulacral primary podia (Figs. 7E, F).DiscussionWe have reported nervous system development in the seacucumber S. japonicus. This is the first detailed report ofnervous system development, from bilateral embryos topentaradial juveniles, in an echinoderm with the ancestral
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