Crabs, Prawns, Shrimps and Lobsters (Class Malacostraca, Order Decapoda) are the most familiar group of crustaceans, and are termed decapods because they have ten legs. The first pair of legs is usually modified for feeding and/or defence, and has a claw. The other four pairs of legs are modified for walking, while in swimmer crabs the last pair is modified as paddles. Crustaceans possess a hard, jointed, external, skeleton for protection, consisting of chitin strengthened by calcium salts. While the skeleton protects the creature inside, it must be periodically discarded to allow the animal to grow. Once the old skeleton is shed, the animal grows rapidly before the soft skin again calcifies and hardens. Hermit crabs have a soft abdomen and utilise a discarded snail shell for protection. Most crustaceans have separate sexes and after internal fertilisation, the female carries the eggs under the abdomen, and the larvae swim away after hatching. In contrast, female prawns release their eggs directly into seawater.