MOTION DIRECTED BY JOINTS. 95 
when the former contracts, the end of the terminal 
segment is brought towards that of the fixed claw. 
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 effected 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, while the basal segment is 
similarly articulated with the thorax. 
If the axes ofall 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 various 
* By axis of the articulation is meant a line drawn through the pair 
of hinges which constitute it. 
