CRAYFISHES AND THEIR RELATIVES 155 



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



through the gill chambers. 



To the Teacher. — Prepare several living crayflsh so 

 that the action of the giU 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. 

 6. 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. Wliere can water enter the gill chamber ? (See 



D, 2, c.) 



8. (Optional.) The gill bailer is a part of one of the cra3rfish'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. 



6. 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 (X4), 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. 



