ADAPTATION TO SURROUNDINGS 77 



mits it ? Where is the body most flexible ? Why ? 

 Where is it most rigid ? Why ? 



4. How much of the surface is covered with exoskele- 

 ton ? What arrangement is there to permit the animal to 

 feel contact ? 



How can the animal grow with such an exoskeleton? 



5. Place a dead crayfish in dilute acid for a few hours. 

 What is the result ? What has the acid done ? Explain 

 the fact that crayfishes are often found alive and well with 

 a soft shell ? 



6. Compare the cephalo-thorax with the abdomen as to 

 size, shape, and flexibility. 



7. How many somites are there in the abdomen ? 

 Which way does it bend ? Study the somite shells on 

 every side and then state what there is in their construc- 

 tion which determines the direction and amount of their 

 motion. How are the somite shells arranged to protect 

 the body during bending .-' How is the ventral surface of 

 the abdomen protected ? 



8. Where are the appendages attached ? Study a walk- 

 ing leg and describe its general construction, the number 

 and kind of joints, the direction of motion in each joint, 

 and the range of motion for the whole leg. Study an 

 antenna in the same way. What methods are used in the 

 crayfish to secure a wide range of motion .■' To secure 

 flexibility .' 



9. Carefully split a crayfish into right and left halves. 

 To do this, first cut through the ventral exoskeleton from 

 end to end with scissors, then with a sharp knife or razor 

 cut through to the dorsal exoskeleton and cut that with the 

 scissors. Study one half, to get a better idea of the 

 attachment of the appendages. These may then be re- 



