CRAYFISH

The body of a decapod crustacean, such as a crab, lobster, or prawn, is made up of nineteen body segments grouped into two main body parts, the cephalothorax, and the abdomen. Each segment may possess one pair of appendages, although in various groups these may be reduced or missing. The cephalothorax is covered by a carapace, which protects the internal organs and the gills; the section of the carapace that projects in front of the eyes is called the rostrum.

On average, crayfish grow to 7.5 centimeters in length. Cambarellus diminutus of the south-eastern United States reaches only about 2.5 centimeters in length, while Astacopsis gouldi of Tasmania may reach 40 centimeters in length and weigh about 3.5 kilograms

Crayfish have a tough exoskeleton, which protects them, but they must molt in order to grow, leaving them vulnerable during this time. The crayfish regularly gets too big for its skeleton, sheds it, and grows a new larger one. Such molting occurs six to ten times during the first year of rapid growth, but less often during the second year. For a few days following each molt, crayfish have soft exoskeletons and are more vulnerable to predators.

Crayfish have two pairs of sensory antennae on the head, and the eyes are on movable stalks. On the thorax or pereon, there are four pairs of walking legs and one pair of pereiopods armed with a claw (pinchers). On the abdomen, or pleon, there are pleopods (also called swimmerets), which are primarily swimming legs. At the end of the pleon is the tail fan, comprising a pair of biramous uropods and the telson, which bears the anus. Together, they are used for steering while swimming, and in the caridoid escape reaction. When they are in danger and need to flee, crayfish can swim backwards quickly by curling and uncurling their abdomen. They breathe through feather-like gills.