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 seawat