AKTHEOPODA. 49 



5. Sight. 



a. Keenness of sight. Watch the movements of a 



crayfish. Does it see objects in its way and 

 turn aside before touching them ? Invent 

 various ways of testing the keenness of sight. 



b. Range of vision. 



(1) What is the extent of the crayfish's range 



of vision? (The crayfish's eye can see 

 you wlierever you can see the crayfish's 

 eye.) 



(2) Can the crayfish see what it is eating ? 



(3) Can it tell where it is going when swim- 



ming ? 



6. Hearing. 



a. Test the hearing of the crayfish. (Record re- 

 sults.) 



7. Circulation of water for Respiration. (Dem.) 



a. Place a crayfish in a shallow dish of water. With 

 a pipette inject a drop of ink into the water 

 near the ventral edge of the carapace. Is the 

 ink drawn under the carapace into the gill- 

 chamber or driven away ? Place another drop 

 between the chelipeds near the mouth. What 

 is the result ? 



h. What is the course of the water through the gill- 

 chamber ? 



c. What causes the water to flow through the gill- 



chamber ? 



d. How are small objects kept out of the gill-cham- 



ber? 



e. How can the crayfish live out of water for 



a considerable length of time as it often 

 does ? 



