i6 



PEOCEEDIKGS OF THE 



any reason, the crab wants to change the direction of its motion 

 — if, for example, it meets an enemy instead of a friend — it can 

 at once, without the slightest pause, reverse its movement. If it 

 wants suddenly to retreat, it has only to lower the two limbs 

 which were previously off the ground, and to raise those which 



Fig. 1. 



Carcinus mcsnas. 



are on the ground, when, by fleiing the limbs on the right that 

 are now in contact with the ground and extending those on the 

 left, it can at once move in the opposite direction. Hence it 

 follows that its course can be at once reversed, owing to the 

 particular rhythm of limb-movement it possesss. 



