MOTION DIRECTED BY JOINTS. 95 



when the former contracts, the end of the terminal 

 segment is brought towards that of the fixed ch^w^ 



A living crayfish is able to perform very varied move- 

 ments with its pincers. When it swims backwards, these 

 limbs are stretched straight out, parallel with one another, 

 in front of the head ; when it walks, they are usually 

 carried like arms bent at the elbow, the "forearm" 

 partly resting on the ground ; on being irritated, the. 

 crayfish sweeps the pincers round in any direction to 

 grasp the offending body ; when prey is seized, it is at 

 once conveyed, with a circular motion, towards the region 

 of the mouth. Nevertheless, these very varied actions 

 are all brought about by a combination of simple flexions 

 and extensions, each of which is eiiected in the exact 

 order, and to the exact extent, needful to bring the chela 

 into the position required. 



The skeleton of the stem of the limb which bears the 

 chela is, in fact, divided into four moveable segments ; 

 and each of these is articulated with the segments on 

 each side of it by a hinge of just the same character as 

 that which connects the moveable claw of the chela with 

 the penultimate segment, wdiile the basal segment is 

 similarly articulated with the thorax. 



If the axes of all these articulations * were parallel, it is 

 obvious that, though the limb might be moved as a whole 

 through a considerable arc, and might be bent in variovis 



* Bj' axis of the arbiculation is meant a liue drawn through the pair 

 of hinges which constitute it. 



