The exoskeleton of crustaceans are hard but flexible except in barnacles which have a calcified shell.  Their body is divided into two major segments.  The first segment is called cephalothorax (head fused with the thorax),  The other segment is the abdomen. The abdomen of most crustaceans consist of many segments.

Most crustaceans are marine and small in size.  These animals are bilaterally symmetrical and most are carnivorous.