Any one of five segments of head (not including anteriormost acron). According to appendages borne, one may distinguish an antennular, antennal, mandibular, maxillular, and maxillar somite. Fused to each other and to thoracic somites to form cephalothorax.
(M. Stachowitsch, 1992)

Alternative form for cephalic somite : cephalomere.