The appendages on the abdomen of the crayfish are called pleopods and uropods. The abdomen has six segments, and the ventral side of the first five segments each have a pair of pleopods or swimmerets. A pair of uropods are present in the sixth segment.
The swimmerets help to maintain water currents over the gills and in reproduction. In the males, the first pair is large, hard and used in sperm transfer. This is used in the determination of sex of the crayfish as all the swimmerets of the female are soft in order to carry the fertilized egg. The paddle-shaped uropod allows the crayfish to move backward when the crayfish is in danger.