Cephalothorax

The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind.

[1] (The terms prosoma and opisthosoma are equivalent to cephalothorax and abdomen in some groups.

The terms prosoma and opisthosoma may be preferred by some researchers in cases such as arachnids, where there is neither fossil nor embryonic evidence animals in this class have ever had separate heads and thoraxes, and where the opisthosoma contains organs atypical of a true abdomen, such as a heart and respiratory organs.

[1] In horseshoe crabs and many crustaceans, a hard shell called the carapace covers the cephalothorax.

[9] The trident is a small group of (usually three) spines found in harvestmen exclusively.