CEAYFISH 77 



3^011 observe and the bubbles, previously seen ? The carmine is 

 used simply to show presence of and direction of water currents. 



7. Why are not air bubbles constantly ejected while the 

 crayfish is in the water ? 



8. Examine a demonstration specimen prepared to show 

 the *' (jin bailer" in action. The "bailer" is a part of the second 

 maxilla. 



9. Make a diagram of a side view of the carapace, and 

 show by arrows the general course of the water currents. Explain 

 your diagram. Briefly summarize the method of aerating the gills. 



10. Observe to what degree the eye stalks are movable. 

 What evidence can you give that the crayfish sees? 



IT. External Ciiaractees 



1. Observe that the body is readily divisible into two re- 

 gions. The large anterior region, which is covered on the dorsal 

 and lateral surfaces by a non-jointed shell called the carapace, is 

 the cephalotliorax. The remaining series of movable segments 

 behind the cephalotliorax is the ahdomen. 



2. Note the cervical groove, a slight depression extending 

 from the ventral margin to the dorsal surface of the carapace. 

 This groove marks the boundary between head and thorax. 



3. Examine the lateral margin of the carapace. This marks 

 the ventral boundary of the gill chamber. On the dorsal surface 

 of the carapace the gill chambers and pericardial chamber are 

 separated by a pair of grooves called the h ranch io-pericardial 

 grooves. The rostrnm is the sharply pointed projection of the 

 cephalothorax between the eyes. 



4. Examine the ventral surface of the cephalothorax. Is 

 there any evidence of segmentation? Notes required. 



5. Note the texture of the skin. Where is it hard and where 

 is it soft? What is the relation of the texture to movability? 

 Notes required. 



