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In binucleid worms, the cephalic segment (also called the "sauce segment" or simply the "head") is a body segment homologous with the first segment of the original Binucleusdetritivorus Worm. In most groups it contains the brain, and in saucebacks it also contains the hip girdle, most of the digestive tract, and the reproductive organs.
Ancestrally, the cephalic segment was protected by two plates, a dorsal plate sometimes referred to as the "sauce" and a ventral plate which has not been given a common name. However, many groups have independently lost one or both of these plates.