<P> Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way . As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth . These nerve cords are connected by transverse nerves like the rungs of a ladder . These transverse nerves help coordinate the two sides of the animal . Two ganglia at the head (the "nerve ring") end function similar to a simple brain . Photoreceptors on the animal's eyespots provide sensory information on light and dark . </P> <P> The nervous system of one very small roundworm, the nematode Caenorhabditis elegans, has been completely mapped out in a connectome including its synapses . Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known . In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and female hermaphrodites, have different numbers of neurons and groups of neurons that perform sex - specific functions . In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons . </P> <P> Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly . Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia . The head segment contains the brain, also known as the supraesophageal ganglion . In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum . Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia . It controls the mouthparts, the salivary glands and certain muscles . Many arthropods have well - developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation . The sensory information from these organs is processed by the brain . </P> <P> In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive--the cell bodies serve only to provide metabolic support and do not participate in signalling . A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals . Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil, in the interior . </P>

The nervous system brain sensory receptors and skin parts originate from this layer of the embryo