The class Malacostraca is divided into five orders, with over 10,000 species the largest is Decapoda. This order also contains the biggest and most familiar crustaceans, such as shrimp, lobsters, crayfish, and crabs.
The name Decapoda means 10 legs, or five pairs of legs. Most decapod's first pair are modified as large claws, called chelipeds. In front of the main legs are three pairs of small appendages used in handling food. These are called maxillipeds, the maxillipeds and five pairs of legs are the appendages of the thorax. A carapace covers the thorax, the head and thorax together are called the cephalothorax. The head has a pair of compound eyes located on eye stalks.
The abdomen is large and has six pairs of biramous appendages. The last pair called uropods are flattened and form a tail fan. Swimmerets or pleopods make up the first five pairs. This structure applies to all decapods except for the crab where the abdomen is reduced and folded beneath the cephalothorax, this is what gives the crab it's short body form.
Diet and Feeding Habits
Most decapods are predators or scavengers, but some feed on algae, and the land crabs eat fruits and leaves. The chelipeds can be used for many different activities, depending on the species. Some crabs use one large claw crushing and the small one for cutting.
There are also filter feeders, such as the mole crabs, and fiddler crabs.
Decapods have gills that project upward from near the base of the thoracic appendages and are enclosed within a protective gill chamber. The gill bailer creates a ventilating current by rapid sculling motion.
The reproductive organs are found in the dorsal part of the thorax. In the male their is a single pair of testes, the sperm ducts open at the base of the fifth pair of legs. In the female the oviducts open to the exterior at the base of the third pair of legs. During copulation the male assumes various positions astride the female and using the greatly modified anterior two pairs of pleopods, transfers spermatophores to the female gonopores or to a median seminal receptacle between the fourth or fifth pair of legs.
Eggs are fertilized internally or on release from the oviduct. Eggs are attached to the pleopods.
Growth and Development
Most decapods hatch as a zoea, although some shrimp hatch as a nauplius. In the zoea all the thoracic appendages are present.
All decapods have the ability to regenerate.