The concept of autotomy is used to describe a defensive mechanism employed by animals to evade predators by way of distracting them or escaping from being captured. The power of regeneration, though not so extensive as is found in annelids, is still pronounced in crayfish. It is however, limited to the regrowth of appendages or portions of them which are broken off or damaged. Sometimes a limb, when caught or held under a stone, or even when stimulated, will be broken off by reflex action of the animal itself. The process is known as autotomy. It has been thoroughly studied in the lobster and probably applies also to the pereiopods of Astacus. The ischipodite of the limb bears a groove near its base. From a point just proximal to the groove two autotomizer muscles are inserted. When they contract, the limb is bent over against a process on the coxopodite and snaps. The breakage plane not damage any muscle, as it lies between the origins of one set and the insertions of the next.
Immediately proximal to the break is a connective tissues membrane stretched across the limb. It has a small perforation in the centre through which the artery and nerve passed down the limb. At autotomy they are withdrawn through this hole into the body. The blood forms a clot over it by cytolysis of special leucocytes and soon the epidermis spreads over the inside. A delicate layer of chitin is formed to protect it. At each moult the stump enlarges until it attains the normal size. The whole process is of distinct survival value; the animal cannot be permanently held by a limb. It is only the more prominent limbs which are likely to be held by a predator and thus autotomy is limited to the chelipeds and pereiopods. Some of the Astacura are known to pull off a damaged or crushed limb.
