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 

 carpal groove, but of greater length. Further, on the inner or proximal side of this groove 

 rises a ridge of lesser arc, which runs in a corresponding groove under the curved ridge 

 of the claw; in brief, circular ridge and groove of claw work on corresponding groove 

 and ridge of fifth segment. To complete this adjustment there is an outgrowth from the 

 hinge process of the carpus, which is outwardly curved, and runs in a corresponding 

 groove distal to the articular ridge on the claw; this serves as an additional lock to the 

 joint, but the proper articular surfaces are those described above. Turning now to the 

 lower or originally anterior side of the claw, we find the conditions completely reversed, 

 and instead of ridge groove we have groove ridge, with corresponding ridge groove on 



Fig. 4. — Locked sliding joint of big claw of lobster. Sectional view 

 of left chela seen from side towards median plane of body, show- 

 ing reversed grooves and ridges of upper and lower hinges. This 

 locked joint is strengthened by the overgrowth of upper and lower 

 hinge processes (uhp and Ih p), which arise from the carpus. 



"These terms are used for the successive segments of the limbs in reference to the median plane of the body. The dacty 

 possesses proximal balls only. 



