. Natural history of the American Decapoda (Crustacea); Lobster fisheries. NATURAL HISTORY OF AMERICAN tOBSTER. 255 The musculature of the great chelipeds is essentially normal and like that of the slender legs, with the exception of the basis or second segment, which has no muscles in the adult state, a condition to be considered in relation to autotomy and the breaking joint; as in the smaller pereiopods the ischium carries two posterior extensors only. The hinges of this limb are quite pecuHar, and suggest possible adaptations to the "breaking joint," and "interloc
. Natural history of the American Decapoda (Crustacea); Lobster fisheries. NATURAL HISTORY OF AMERICAN tOBSTER. 255 The musculature of the great chelipeds is essentially normal and like that of the slender legs, with the exception of the basis or second segment, which has no muscles in the adult state, a condition to be considered in relation to autotomy and the breaking joint; as in the smaller pereiopods the ischium carries two posterior extensors only. The hinges of this limb are quite pecuHar, and suggest possible adaptations to the "breaking joint," and "interlock," considered in a later section. In place of anterior balls working in posterior sockets, as in the tail, we have proximal balls moving in distal cups," with the exception of the first, fifth, and sixth podomeres, for the hinges between the carpus and big claw are so peculiar that they merit special attention. As we have seen, the order in the hinges of the basal joints of all the thoracic appendages is socket and ball of limb, united to ball and socket of the body. LOCK HINGES OF BIG CLAWS. By far the most peculiar joint and one of the most unique mechanical devices in the lobster's skeleton are the concealed, sliding hinges, by means of which the great forceps are securely locked and articulated to. (ffTvove) propodus the rest of the Umb. By referring to plate (xxxvii and text fig. 4) it will be seen that the great claw swings between flattened processes of the carpus, which embrace the upper and lower sides of its proximal end near the joint. These two processes (w and I h p) conceal the joint in question, and lock the claw firmly to the carpus, upon which it is free to move in the horizontal plane through an arc of about 135°, but from which it can not be removed without breaking either segment. When the hard shell is broken at this joint the upper hinge on the claw side is seen to consist of a prominent semicircular ridge, which fits into a corresponding carpa
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