The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas like real segments do. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the uropods, the telson forms the tail fan of lobsters, shrimp and other decapods. These are used as a paddle in the caridoid escape reaction ("lobstering"), whereby an alarmed animal rapidly flexes its tail, causing it to dart backwards. Krill can reach speeds of over 60 cm per second by this means. The trigger time to optical stimulus is, in spite of the low temperatures, only 55 ms.

The same term telson is widely used (e. g. invertebrate textbooks such as Ruppert & Barnes, 1994 and Brusca & Brusca, 2003) for the caudal spine of Chelicerata, clearly seen in a number of fossil species and for example in the horseshoe crabs and the scorpion sting. Usage of this word in this context is discouraged. Lauterbach (1980) provided a fine discussion on this, with many illustrations.