The Japanese Giant Spider Crab has two central nervous system units. One is located at the front of the crab, one at the back. They are called the dorsal ganglion and the ventral ganglion. The dorsal ganglion is the brain. It is very, very tiny. Smaller than the tip of a pencil. The ventral ganglion is huge by comparison. The brain processes what the eyes see, while the ventral ganglion controls the legs and claws. It is the largest arthropod in the world.