Endocrinology of protochordates (NRC)
This was written before the amphioxus genome had been sequenced, so it's not up to date. However, the article presents considerable information derived from cDNA analysis.
Although ostensibly about hormones, the article also summarizes the neurotransmitters found in protochordates. They are: Acetylcholine, GABA, Adrenaline, Noradrenaline, Serotonin, Dopamine, Glutamate, Glycine, Aspartate and Taurine.
For a more complete discussion of the amino acid neurotransmitters in amphipoxus, see the paper, below:
Free amino acids in the nervous system of the amphioxus Branchiostoma lanceolatum. A comparative study (Goog)
Discussed in Amphioxus Neurotransmitters.
Serotonin localization in Phallusia mammillata larvae and effects of 5-HT antagonists during larval development (Goog)
Full length PDF available online for free.
"Using immunofluorescence with an antiserotonin antibody, 5-HT was localized in the brain and in some neurons of the larval tail of Phallusia mammillata."
"The origin of 5-HT-containing cells during the ontogeny
of protochordates is not so well known as in vertebrates
and sea urchins. Localization of 5-HT has been demonstrated in 2-day-old larvae of the amphioxus
(Holland & Holland 1993)." The Holland reference is excerpted in Amphioxus Neurotransmitters .
Phallusia mammillata (Goog)
"The largest sea squirt in the British Isles."