The cephalothorax and head are fused into a single unit. It has an oval body with extremely long legs, which they frequently lose in various accidents and brushes with predators. They are predators, herbivores, and scavengers. Look at them closely next time you see one. Despite their aquatic diversity, there are very few terrestrial crustaceans, just as there are very few truly aquatic insects.
Crustaceans have biramous appendages .
Each leg has an additional process, like a little miniature leg branching off from the main leg. Many groups of crustaceans have lost this extra appendage during subsequent evolution. The Order Decapoda have five pair of walking legs, and include the familiar crabs, lobsters, and crayfish. The first pair of appendages are usually modified as antennae . Crustaceans have two pair of antennae.