Copepods are a diverse and ecologically crucial group of minute crustaceans that 
are relatively neglected in terms of studies on nervous system organization. 
Recently, morphological neural characters have helped clarify evolutionary 
relationships within Arthropoda, particularly among Tetraconata (i.e., 
crustaceans and hexapods), and indicate that copepods occupy an important 
phylogenetic position relating to both Malacostraca and Hexapoda. This taxon 
therefore provides the opportunity to evaluate those neural characters common to 
these two clades likely to be results of shared ancestry (homology) versus 
convergence (homoplasy). Here we present an anatomical characterization of the 
brain and central nervous system of the well-studied harpacticoid copepod 
species Tigriopus californicus. We show that this species is endowed with a 
complex brain possessing a central complex comprising a protocerebral bridge and 
central body. Deutocerebral glomeruli are supplied by the antennular nerves, and 
a lateral protocerebral olfactory neuropil corresponds to the malacostracan 
hemiellipsoid body. Glomeruli contain synaptic specializations comparable to the 
presynaptic "T-bars" typical of dipterous insects, including Drosophila 
melanogaster. Serotonin-like immunoreactivity pervades the brain and ventral 
nervous system, with distinctive deutocerebral distributions. The present 
observations suggest that a suite of morphological characters typifying the 
Tigriopus brain reflect a ground pattern organization of an ancestral 
Tetraconata, which possessed an elaborate and structurally differentiated 
nervous system.