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. 36, 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 ihepleuron (fig. 36, pi. XIX). 



