578 K. EAY LANKESTEK. 



angle formed by the limb and the long axis of the body, 

 appears to proceed from behind forwards in all three cases. 

 Each leg may be considered as resting normally at right 

 angles to the axis of the body. Each is capable of a certain 

 forward swing in the horizontal plane, being provided with 

 a joint and muscles at its base, and of a corresponding 

 backward swing in Avhich the leg passes its first position 

 (that perpendicular to the axis of the body) and makes an 

 excursion or deflection away from the perpendicular in the 

 posterior direction. When the animal is in a state of 

 locomotor activity all the legs steadily swing forwards and 

 backwards through their extreme range of angular displace- 

 ment, each at the same rate. But they do not all simul- 

 taneously assume the same angular position relatively to the 

 axis of the body, nor, on the other hand, do they swing 

 irregularly. They pass consecutively^ from behind forwards 

 into an identical phase of the swing movement, the leg in 

 front taking up the angular phase just previously exhibited 

 by the leg behind it, which in the meantime has continued 

 its swinging movement, either becoming more deflected or 

 now commencing the return movement. The rate of swing 

 is such that in all cases as yet observed not one great wave 

 occurs but a series of waves are produced, as when wind 

 blows over a cornfield. These waves vary in the number of 

 units (legs) involved in a complete wave according to the 

 kind of Arthropod or Chtetopod under observation. The 

 number of units involved in a ''wave" or ''swing-group" 

 seems to be fixed in a given species, and not to vary accord- 

 ing to circumstances. Whether the rate or relative rate of 

 forward swing is always the same as that of the backward 

 swing (which is that portion of the swing effective in pro- 

 pulsion) has yet to be ascertained, as also the exact excursion 

 made on each side of the perpendicular. Also it would be 

 interesting to ascertain what are the limits of increase and 

 diminution of the rate of swing, and what nervous mechanism, 

 if any, is concerned in its regulation. 



These phenomena can only be studied satisfactorily by 



