At the posterior end of this shield is a stiff tapering tail. Thus this animal is completely covered on the upper surface exposing its eyes alone to the view. It has three pairs of walking legs with clawed extremities and a pair of legs with flattened oar-like toes.

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The body of a king crab consists of a Cephalothorax, which is covered by a horseshoe-like case and a segmented abdomen, which is covered by an abdominal shield.

King crabs are found along the sea coasts of many countries. They live in moderately deep waters on a sandy bottom, 2 to 8 fathoms down. They come to the shallow waters to spawn during spring season in temperate regions.

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The earlier naturalists, judging it by its horny shield, awkward movements, marine habitat and breathing gills, called it a king crab or horseshoe crab, although it is really an arachnid.

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King crabs are active during the day, they lie partly buried in mud or sand. They feed on sea worms and mulluscs. They hunt their prey by burrowing. The female king crabs are larger than the males. Though they look dreadful, they are all harmless to man.

Two species of king crabs called Limulus are common on the coast of Bengal. The freshly hatched king crabs look like small adult king crabs but without the tail.