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 the ordinary ball-and-socket device of the other limb segments, and 

 even more that of the crab's chela, it would be difiicult to decide whether one was 

 better from a mechanical standpoint than the other, or to imagine how either could 

 have arisen from the simpler type upon any principle of selection. 



(socket^ 



Itvp 



propodus 



Fig. 5. — Locked sliding joint of big claw of crab (Catlinecles hastatus); 

 in same plane as represented in figure 3 , showing modified balls and 

 sockets, laut witli no reversal on upper and lower sides; hinge processes 

 (m and I h p) here arise from the propodus of claw. 



ASYMMETRY IN THE BIG CLAWS OF THE LOBSTER. 



The marked dissimilarity of the big claws (pi. xxxvii) in regard to both their 

 structure and chief functions in all lobsters above an inch or an inch and one-half long, 

 has led to various distinctive names on both sides of the Atlantic. Fishermen often 

 speak of the "knobbed" and "quick" claws. The larger is adapted for crushing the 



