Caenorhabditis elegans
C. elegans is a roundworm approximately 0.75mm in length. It's nervous system is an odd one because it doesn't actually have a central nervous system. It has packets of nerves located in the head and tail, and tubular bundles of nerves called the dorsal and ventral cords. C. elegans is particularly important to science because it was the first, and only organism to have it's entire neural network mapped, down to the level of individual neurons. To see an interactive map of the C. elegans neural network, click the button below: