Ventral nerve cord

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The ventral nerve cords make up the nervous system of arthropods. It usually consists of (head to tail):

head ganglia (brain, frontal and suboesophageal ganglion)
thoracic ganglia (3 in insects)
abdominal ganglia

The two ventral cords from thorax to abdomen are made up of segmental ganglia that are intra-segmentally connected by commissures and inter-segmentally by connectives. The complete system bears some likeness to a rope ladder. Sometimes the paired segmental ganglia plus their two connectives are fused into one large ganglion per segment, which is the case for most insects. Although, strictly speaking, the head ganglia are not usually identified with the paired cords in the lower segments, the terms arthropod nervous system and ventral nerve cords are mostly used interchangeably.