Cross references: Hemichordates Hemichordate Hormones
Hemichordate Nerves Hemichordate Genomics
Hemichordate Larva (Tornaria)
Development of the enteropneust Ptychodera flava (PubMed) - 2007
"The serotonergic nervous system of the fully grown tornaria consists of an apical ganglion with many perikarya, a paired lateral group of perikarya on the postoral ciliary band, and scattered perikarya along the opisthotroch. Serotonergic processes are found along the ciliary bands except for the ventral and perianal ciliary bands and are scattered along the epidermis.
At the Spengel stage and at metamorphosis (Agassiz stage), the processes along the ciliary bands are concentrated in the three ciliated food grooves so as to form three separate nerves, and are retained on the proboscis at least until 2-3 gill slit stage.
No serotonergic processes were found to extend from the proboscis to the collar region, and no serotonergic neurons were observed in the collar cord or in the ventral nerve cord."
I'm very interested in the serotonergic nervous system. However, the tornaria larva is so different from the adult acorn worm that I am unable to extrapolate from this description of serotonin in the larva to serotonin in the adult.
See: Hemichordate Larva (Tornaria)