The hermit crab

An arthropod whose bilogical classification is some where between the crabs and crayfish. While the upper parts of their body is equipped with strong armour and powerful claws, it seems that nature overlooked the rear part. As soon as the pagurus hatches out it digs a trench in the sand in which it hides its unprotected abdomen, leaving from time to time to quickly scuttle across the sea floor, looking for an empty shell that a misfallen mollusc left behind.