Mapping behavioral data and neuronal characters on the current nematode phylogeny. To understand the possible evolution of modulated behaviors and biogenic amine neurons, we overlaid our data on a current phylogenetic tree. We observe that most of the Eurhabditids exhibit both basal slowing and enhanced slowing responses with the exception of R. axei. Outside of the Eurhabditids, neither of the other species tested exhibited a basal or enhanced slowing response in our experiments. The number of dopaminergic neurons was essentially unchanged among the different species examined. For the serotonin-immunoreactive neurons, the NSM neurons are conserved across all species; conservation of other cells is less certain. A presumptive ADF neuron is found in several species; overall, the number of serotonergic neurons varies considerably among the nematode species tested.