CRAYFISHES AND THEIR RELATIVES 155 



7. How the crayfish causes a current of water to pass 



through the gill chambers. 



To the Teacher. Prepare several living crayfish so 

 that the action of the gill bailer may be seen. To do 

 this carefully cut off a small part of the anterior por- 

 tion of the shell just over the gill scoop. 



Watch the movements of the small blade-like body 

 in the front of the gill chamber. This body is 

 the gill bailer, or gill scoop. 



a. Describe the movements of the gill scoop, or gill 



bailer. 



b. When it moves upward and forward, what effect 



will the gill bailer have on the water in front 

 of it and in the gill chamber ? 



c. Where can water enter the gill chamber ? (See 

 D, 2, c.) 



8. (Optional.) The gill bailer is a part of one of the crayfish's 



mouth parts, known as the second maxilla. Exam- 

 ine a second maxilla that has been removed from 

 the head thorax of a preserved crayfish. Place it 

 in a watch glass half filled with water and make out 

 the following parts : 



a. A part shaped something like a bird's wing, composed of 



several pieces. 



b. The gill bailer that you have already seen. 



c. The part where the second maxilla was torn from the 



body, clearly shown by the shreds of muscle. 

 When you* have made out these parts, make a sketch 

 of the second maxilla (X 4), and label: winglike 

 part, gill bailer, shreds of muscle. 



9. How does the shape of the gill bailer fit it for the work it does ? 



E. (Optional.) Adaptations for food getting. 



1. Place an earthworm, a piece of beef, or a piece of clam near a 

 crayfish, and describe the way in which he gets the 

 food to his mouth. 



