Crustaceans
Crustaceans is a group of arthropods that includes crayfish, lobsters, crabs, and shrimp. Most have a body with three parts - head, thorax, and abdomen - but in some species the head and thorax are joined. All live in the water, except wood lice and their relatives.
Crayfish has eight walking legs and two large claws. It flicks its tail in order to swim. Crabs are decapods (creatures with 10 legs). Most live in or near water, though some crabs remain on land until they lay their eggs. They are mostly scavengers, surviving by eating dead animals and plants.
The world’s largest crustaceans are the giant spider crabs of the northern Pacific Ocean. They have a fairly small body, but their legs can span a distance of 8 feet (2.4 m).
Mantis shrimps kill crabs and mollusks by smashing them open with a pair of clublike legs. The mantis shrimp’s blows are so powerful that they have been known to break the glass walls of aquarium tanks.