THE CRAYFISH AND ARTHROPODS IN GENERAL 221 



amount of tissue removed. If one chela is amputated, a new 

 chela regenerates less rapidly than if both chelae and some of the 

 other walking legs are removed (189). 



Behavior. When at rest, the crayfish usually faces the en- 

 trance to its place of concealment, and extends its antennae. 

 It is thus in a position to learn the nature of any approaching 

 object without being detected. Activity at this time is reduced 

 to the movements of a few of the appendages and the gills; the 

 scaphognathites of the second maxillae move back and forth baling 

 water out of the forward end of the gill chambers; the swimmer- 

 ets are in constant motion creating a current of water; the maxil- 

 lipeds are likewise kept moving; and the antennae and eye stalks 

 bend from place to place. 



Crayfishes are more active at nightfall and at daybreak than 

 during the remainder of the day. At these times they venture 

 out of their hiding places in search of food, their movements 

 being apparently all utilitarian and not for spontaneous play or 

 exercise (167). 



LOCOMOTION. Locomotion is effected in two ways, walking 

 and swimming. Crayfishes are able to walk in any direction, 

 forward usually, but also sidewise, obliquely, or backward. In 

 walking, the fourth pair of legs are most effective and bear nearly 

 all of the weight of the animal ; the fifth pair serve as props, and 

 to push the body forward; the second and third pairs are less 

 efficient for walking, since they are modified to serve as prehensile 

 organs, and as toilet implements (168). Swimming is not re- 

 sorted to under ordinary conditions, but only when the animal is 

 frightened or shocked. In such a case the crayfish extends the 

 abdomen, spreads out the uropods and telson, and, by sudden 

 contractions of the bundles of flexor abdominal muscles, bends the 

 abdomen and darts backwards. The swimming reaction appar- 

 ently is not voluntary, but is almost entirely reflex (168). 



EQUILIBRATION. The crayfish either at rest or in motion is in 

 a state of unstable equilibrium, and must maintain its body in the 

 normal position by its own efforts. The force of gravity tends 



