Crustaceans are a very ancient and diverse group. Originally they expanded to occupy every possible situation in all the world's oceans and are still more numerous and varied than the insects on land. More than 30,000 living species have been described (and many, many more from fossils). They range in size from the microscopic to giant deep sea crabs with leg spans of more than 4m. and lobsters weighing up to 8 kg. They are all arthropods, that is, animals with hard exoskeletons and numerous pairs of jointed limbs. This group includes such familiar creatures as Sea Monkeys or Brine Shrimps (used as aquarium food), prawns, shrimps, crabs, terrestrial slaters or wood lice, and many freshwater forms such as yabbies and crayfish. In the oceans one group of small crustacean called copepods reign supreme as the 'insects of the sea'.

• A hard chitinous jointed exoskeleton, sometimes calcified for strength. Often a hard carapace covers the head, thorax and gills
• A many-segmented body basically divided into 3 parts: head, thorax (often fused as a cephalothorax) and abdomen.
• Jointed appendages which are biramous (ie have 2 branches variously modified for a particular function).
• A head with 5 segments with limbs modified as 2 pairs biramous antennae, (unique to crustacea), mandibles (jaws) and 2 pairs maxillae.
• A thorax with numerous pairs of walking/swimming legs with 2nd branch used as gills for gas exchange.
• To grow by molting (shedding of old exoskeleton and expanding into a new one).