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Abdomen
The segmented section of the crayfish, called the abdomen, has a series of muscles running up and down the sides which expand and contract, causing a flicking forward and downward motion that allows the tail fan to thrust the crustacean backwards.
Swimmerets
Five pairs of filamentous leglike appendages found under the abdomen behind the last pair of walking legs are the swimmerets, which are thinner and smaller than the walking legs. Called pleopods, four pairs of these appendages synchronize strokes that help the crayfish swim forward under normal circumstances, and all five pairs create water currents that aid the tail fan to whisk the animal backwards in an emergency. The swimmeret has three sections, the protopodite, which is like a muscular upper arm; it supports two endopodites that have cilia or hair that aids in moving the water.
Tail Fan
The strong region at the end of the last section of abdomen of the crayfish, the tail fan, has a high surface area that acts like an edge of an oar or paddle. The flicking of the tail fan, when tucked up and under the abdomen when in escape mode, is called the tail-flip which begins to respond to a stimulus in about two-hundredths of a second. When the tail-flip is triggered, all other behaviors that may interfere with it are cancelled and are instantly reinstated afterwards.
Telson
The central most posterior part of the tail fan is the telson; this contains the anus and works in conjunction with the uropods, forcing the water forward and shooting the animal backwards.
Uropods
On either side of the telson is a pair of anterior flippers, called uropods. These structures feature three sections, the protopodite, the lateral exopodite and the medial endopodite. These are similar to the sections of the swimmerets, but these units are larger, flatter and broader, and not only aid in the thrust, but steer the crayfish during the caridoid escape reaction.