. Natural history of the American Decapoda (Crustacea); Lobster fisheries. 256 BULLETIN OF THE BUREAU OF FISHERIES. the lower hinge process of the carpus. It follows from these relations that the articular surfaces of the carpus face, while those of the claw look in opposite directions. This remarkable joint suggests the hinge of an ordinary folding pocket rule, but with a different locking device. It is neither a true pivot, tenon-and-groove, or ball-and- socket joint, and so far as I am aware its principle is not found embodied in any of the common mechanical devices. We find it w
. Natural history of the American Decapoda (Crustacea); Lobster fisheries. 256 BULLETIN OF THE BUREAU OF FISHERIES. the lower hinge process of the carpus. It follows from these relations that the articular surfaces of the carpus face, while those of the claw look in opposite directions. This remarkable joint suggests the hinge of an ordinary folding pocket rule, but with a different locking device. It is neither a true pivot, tenon-and-groove, or ball-and- socket joint, and so far as I am aware its principle is not found embodied in any of the common mechanical devices. We find it well developed at the fourth stage, with little later change except in the further overgrowth of the hinge processes. (Fig. 9.) Such a joint works with great precision in its prescribed plane, with little or no appreci- able lost motion, and would seem to be an adjustment by means of which the big claw is firmly secured to the supporting carpus, and the voluminous flexors of this segment can react upon the great weight of the claw to the best advantage. In the crayfish (Cambarus) the big claw is not locked to the carpus, but moves loosely on double hinges of the typical ball-and-socket order, each hinge consisting of carpal ball, and propodal socket mounted on a round tubercle. In Callinectes and certain other Brachyura exam- ined (text fig. 5) the great cheliped has suffered little or no torsion, and the dactyls open upward as in the larval lobster. The claws move on modified ball-and-socket hinges, which are firmly locked to the claw but in quite a different manner from that of the lobster. The propodus in this case bears cups (/ h (socket) fig. 5) on both upper and lower sides, which are locked over the balls by processes (u and I h p) growing out from this segment and not from the carpus. The crab's claw thus swings vertically in and out through an angle of upwards of 90°. While the locked, sUding joint of the lobster, particularly in the reversal of its hinges, suggests th
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