94 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



articulation of the last joint (fig. 20, dp) with the one which 

 precedes it (j5rp) is examined, it will he found that the 

 base of the terminal segment {dp) turns on two hinges {x), 

 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 tha 

 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 (t) ; 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 (<'). 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. 



