152 RE GENERA TION 



of the leg; for the leg of a dead crab will support a weight of 3^ 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. 



In many crabs the leg is not thrown off if simply held, but only 

 after an injury. Even the most distal segment may be cut off and 

 the leg remain attached, and sometimes it is not lost after the distal 

 end of the next to the last segment is cut off. If a crab is tethered 

 by one leg it will not throw off its leg in order to escape, unless, in 

 the crab's excitement, the leg is twisted or broken. How generally 



