I'm afraid I can only give you hint. I am quite certain that the
sea-slug Aplysia californica is the organism you are looking for. Of
course it has a few more neurons than nine, but the nervous system as a
whole seems to be quite well understood and many behavioural aspects are
mapped to certain groups of neurons.
I am sorry, that I can't provide more information, but I can't find
where I read the basics on Aplysia. You will definetly find a lot of
behavioural aspects and even more about learning and memory of Aplysia
in any Neuroscience textbook. That should be a good starting point. Of
course a PubMed search will probalby also bring more material than one
person could read in a month :-)
Concerning the nine neurons of the horseshoe crab...I find that hard to
believe. Possibly that number refers to a specific neural net within the
horseshoe crab, which is well understood...
Chr. Wilms (melvin at gmx.de)