Abstract
The nervous system development of the sea cucumber Stichopus japonicus was investigated to explore the development of the bilateral larval
nervous system into the pentaradial adult form typical of echinoderms. The first nerve cells were detected in the apical region of epidermis in the
late gastrula. In the auricularia larvae, nerve tracts were seen along the ciliary band. There was a pair of bilateral apical ganglia consisted of
serotonergic nerve cells lined along the ciliary bands. During the transition to the doliolaria larvae, the nerve tracts rearranged together with the
ciliary bands, but they were not segmented and remained continuous. The doliolaria larvae possessed nerves along the ciliary rings but strongly
retained the features of auricularia larvae nerve pattern. The adult nervous system began to develop inside the doliolaria larvae before the larval
nervous system disappears. None of the larval nervous system was observed to be incorporated into the adult nervous system with
immunohistochemistry. Since S. japonicus are known to possess an ancestral mode of development for echinoderms, these results suggest that the
larval nervous system and the adult nervous system were probably formed independently in the last common ancestor of echinoderms.
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