Réaumur first recorded that if the leg of a crayfish or of a crab is cut off outside of the breaking-joint it is always thrown off later at the breaking-joint. Fredericq has made a careful examination of the way in which the leg is thrown off in the crab, _Carcinus mænas_. He found that the breaking does not take place at the weakest part of the leg; for the leg of a dead crab will support a weight of 3½ to 5 kilograms, which represents about one hundred times the weight of the crab’s body. When the weight is increased to a point at which the leg breaks, it does so between the body and the first segment or between the first and second segments. When it breaks off in this way, the edges are ragged and are left in a lacerated condition; but when the leg is thrown off by the animal at the breaking-joint, there is left a smooth surface covered over, except in the centre, by a thin cuticle. Through the opening in the centre of this cuticle a nerve and a blood vessel pass to the distal part of the leg. Very little bleeding takes place when the leg is thrown off, but if the leg is cut off or broken off at any other level the bleeding is much greater. Fredericq studied the physiological side of the process and found that it is the result of a reflex nervous act. He found that if the brain of the animal is destroyed the leg may still be thrown off, but if the ventral cord is destroyed the reflex action does not take place. The reflex is brought about ordinarily by an injury to the leg that starts a nerve impulse to the ventral nerve-cord, and from this a returning impulse is sent to the muscles of the same leg, causing the muscles in the region of the breaking-joint to contract violently, and the result of their contraction is to break off the leg. If the muscles are first injured, the leg cannot be thrown off. Andrews, who has studied the structure of the breaking-joint in the spider-crab; has found that there is a plane of separation extending inwards from the groove on the surface. This plane is made by a narrow space between two chitinous membranes that are continuous at their outer ends with the general chitinous covering of the leg (Fig. 45, _C_). When the leg breaks off, one-half of the double membrane is left attached to the base of the leg and the other to the part that is lost. This in-turned membrane seems to correspond to the in-turning of the surface cuticle in the region of the joints. The arrangement of the muscles at the breaking-joint is shown in Fig. 45, _B_. The upper muscle is the extensor muscle of the leg, and through its contraction the breaking off takes place. When it contracts the leg is brought against the side of the body, which is supposed to offer the resistance necessary to break off the leg. If the leg is held by an enemy, this may offer sufficient resistance for the muscle to bring about the breaking.