• Limulus is known as the ‘King crab’, it is a Living fossil.

• Marine, burrowing animal.

• Body consists of prosoma and opisthosoma.

• Prosoma bears a pair of simple median eye and a pair of lateral compound eyes.

• Prosoma bears a mouth; it has 6 pairs of appendages.

• Opisthosoma has 6 pairs of lateral spines, 6 pairs of appendages and caudal spines.

• Respiratory organs are book gills.

• Excretory organs are coxal glands.

• Egg hatches into trilobite larva.