90 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 neaii}' 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 probabl}" 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 seizetl, the experimenter will find, to his 

 cost, that the animal solves the problem both rapidly 

 and accurately. 



The mechanism b}- 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 stern inn (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 the pleuron (fig. 36, pi. XIX). 



