In the Penaeidae the head (five somites) and thorax (eight somites) are fused into a cephalothorax, which is completely covered by the carapace. The pleura of the cephalothorax form the branchiostegite or gill cover. The carapace has characteristic ridges (carinae) and grooves (sulci). The rostrum is always prominent, with a high median blade bearing dorsal teeth and, in some genera, ventral teeth as well. The compound eyes are stalked and laterally mobile and the somites of the head bear, in order, pairs of antennules, antennae, mandibles, maxillules (maxillae 1) and maxillae (maxillae 2). The thorax has three pairs of maxillipeds and five pairs of pereopods (legs), the first three being chelate and used for feeding, and last two simple (non-chelate) and used for walking. The abdomen consists of six somites, the first five with paired pleopods and the sixth with uropods that. The mouth is situated ventrally and the cephalic appendages surrounding it, plus the first and second maxillipeds and sometimes the third as well, may be referred to collectively as the "mouth parts". The anus is on the ventral surface of the telson towards its base (Dall et al., 1990).