There are a few terrestrial groups of crustaceans (such as terrestrial crabs, terrestrial hermit crabs and woodlice) as well as a handful of parasitic groups (such as tongue worms, whale lice and fish lice). Most crustaceans are free-living but some (such as the barnacles) are sessile. The crustacean body is divided into segments (or tagmata) which include a head, a thorax and an abdomen. In some crustaceans, the head and thorax are fused into a structure referred to as the cephalothorax.