NATURAL HISTORY OF AMERICAN LOBSTER. 
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 peculiar, 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 OP 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 
the rest of the limb. 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 (u and / 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 0 , 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 
a 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. 
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 (u h p and l h p), which arise from the carpus. 
