94 THE PHYSLOLOGY OF THE COMMON CRAYFISH. 
articulation of the last joint (fig. 20, dp) with the one which 
precedes it (prp) is examined, it will be found that the 
base of the terminal segment (dp) turns on two hinges (a), 
formed by the hard exoskeleton and situated at opposite 
points of the diameter of the base, on the penultimate 
segment; and these hinges are so disposed that the 
last joint can be moved only in one plane, to or from 
the produced angle of the penultimate segment (prp), 
which forms the fixed claw of the chela. Between the 
hinges, on both the inner and the outer sides of the 
articulation, the exoskeleton is soft and flexible, and 
allows the terminal segment to play easily through a 
certain arc. It is by this arrangement that the direction 
and the extent of the motion of the free claw of the chela 
are determined. ‘The source of the motion lies in the 
muscles which occupy the interior of the enlarged penul- 
timate segment of the limb. Two muscles, one of very 
great size (m), the other smaller (m’), are fastened by 
one end to the exoskeleton of this segment. The fibres of 
the larger muscle converge to be fixed into the two sides —- 
of a long flat process of the chitinous cuticula, on the 
inner side of the base of the terminal segment, which 
serves as a tendon (¢) ; while those of the smaller muscle 
are similarly attached to a like process which proceeds 
from the outer side of the base of the terminal seg- 
ment (t’). It is obvious that, when the latter muscle 
shortens it must move the apex of the terminal seg- 
ment (dp) away from the end of the fixed claw; while, 
