cephalothorax In some members of the phylum Arthropoda, the fused head and thorax. It is found in members of the Chelicerata (classes Merostomata, Arachnida, and Pycnogonida), and in most Crustacea. In spiders, it is the prosoma, which is covered by a sclerotized plate (carapace), and which bears the walking limbs, pedipalps, chelicerae, and eyes. The opisthosoma (abdomen) is joined to the cephalothorax by a stalk (pedicel).

ceph·a·lo·tho·rax / ˌsefəlōˈ[unvoicedth]ôraks/ • n. (pl. -tho·ra·ces / -ˈ[unvoicedth]ôrəˌsēz/ or -tho·rax·es) Zool. the fused head and thorax of spiders and other chelicerate arthropods.

cephalothorax The fused head and thorax of many crustaceans and of arachnids (see prosoma), which is connected to the abdomen. See also tagma.