96 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
degrees, yet all its movements would be limited to one 
plane. But, in fact, the axes of the successive articula- 
tions are nearly at right angles to one another; so that, 
if the segments are successively either extended or 
flexed, the chela describes a very complicated curve ; 
and by varying the extent of flexion or extension of 
each segment, this curve is susceptible of endless varia- 
tion. It would probably puzzle a good mathematician 
to say exactly what position should be given to each 
segment, in order to bring the chela from any given 
position into any other; but if a lively crayfish is 
incautiously seized, the experimenter will find, to his 
cost, that the animal solves the problem both rapidly 
and accurately. 
The mechanism by which the retrograde swimming of 
the crayfish is effected, is no less easily analysed. The 
apparatus of motion is, as we have seen, the abdomen, 
with its terminal five-pointed flapper. The rings of the 
abdomen are articulated together by joints (fig. 21, x) 
situated a little below the middle of the height of the 
rings, at opposite ends of transverse lines, at right 
angles to the long axis of the abdomen. 
Each ring consists of a dorsal, arched portion, called the 
tergum (fig. 21; fig. 36, p. 142, t. XIX), and a nearly flat 
ventral portion, which is the sternum (fig. 86, st. XIX). 
Where these two join, a broad plate is sent down on 
each side, which overlaps the bases of the abdominal 
appendages, and is known as the pleuron (fig. 36, pl. XIX), 
