The second to fourth set legs are used for walking and gathering food. The tips of these legs form slender long pinchers which grab food and move it to the mouth. The last set of legs have specialized ends for pushing in soft sediment. The opisthosoma is the triangular shaped region of the crab. The surface structures in this region include the book gills and reproductive organs.
Book gills are the respiratory organ of the crab and are also used in locomotion.
There are six sets of gill flaps associated with the book gills. The first flap called the operculum contains no gills. It covers and protects the gills and reproductive organs. Each remaining flap has around 80 gill membrane sheets used in respiration. The opisthosoma has two flanges at its base on ether side of the telson.