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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 as do real segments. 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.

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References

References

★ Richard C. Brusca & Gary J. Brusca. 2003. Invertebrates. 2nd edition (January 2003), Sinauer Associates. 936 pages.

★ Lauterbach, Karl-Ernst. 1980. Schüsselereignisse in der Evolution des Grundplans der Arachnata (Arthropoda). ''Abh. naturwiss. Ver. Hamburg'' (NF) 23: 163-327.

★ Ruppert, E.E. & R.D. Barnes. 1994. Invertebrate Zoology, 6th Ed, Saunders.