56 GENERAL ZOOLOGY 



it rushes forth, seizes the intruder and tries to eat it. When 

 hunger is satisfied the crayfish sits in its lair without any 

 movement, except for respiration, for days at a time. Its 

 activities are for the most part strictly utilitarian. Yet a 

 crayfish has some ability to profit by experience. If a large 

 shadow is cast over one repeatedly, it first avoids the shadow 

 at each repetition and puts itself in an attitude for defense, 

 but finally learns to ignore this new thing which has come 

 into its life. Professor Yerkes taught a crayfish to go 



Fio. 30. A young crayfish attached to a portion of one of its mother's 

 abdominal appendages. Until after its second moult the young crayfish is 

 firmly fastened by an "anal filament." (Adapted from Andrews.) 



through a simple labyrinth where it had equal chance to 

 enter a blind alley or to go through an opening into the 

 water. At first it took the wrong course half of the time, 

 but after two hundred and fifty trials made no more 

 mistakes. 



If we compare a crayfish with the ancestral arthropod 

 described at the beginning of this chapter, we see that it 

 is specialized in several respects. The abdomen is the 

 most primitive part of the body, being clearly metameric 

 with a pair of biramous appendages on each segment. The 

 thorax is overgrown by the great carapace and shows evi- 



