- The definition of a cephalothorax is the head and chest on crustaceans and arachnids.
An example of a cephalothorax is the head and chest of a crab.
the head and thorax united as a single part, in certain crustaceans and arachnids
The anterior section of arachnids and many crustaceans, consisting of the fused head and thorax.
(plural cephalothoraxes or cephalothoraces)