The telson of a crayfish is the center segment of the arthropod's tail, which is the furthest back extension of the abdomen. The telson, in conjunction with the other segments of the tail, aids the creature in locomotion through water.
The crayfish is a freshwater crustacean whose anatomy is made up of segmented exoskeletal sections. The animal is a decapod, which means that it has 10 legs, and has a tail section that extends from the abdomen. Each pair of legs is connected to a segment of the body called a swimmeret, which allows the creature to walk along the lake or river floor. Behind the swimmerets, there are two segments referred to as uropods, and the telson sits behind the uropods as the back-most body segment.
Unlike each of the segments in front of the telson, the telson is not a paired segment. Instead, it is one large segment that makes up the whole tail of the creature that includes the anus and the tail fan. The telson can be used to propel the animal forward through the water, much like a fin, but it is also an important piece of the body because it serves as a shield for the eggs of the female crayfish.