70 INTRODUCTION TO ZOOLOGY. 



14. Find the eyes. Are they simple or compouud? 

 See how they can move. Are tliey on stalks, or 

 are they sessile? 



15. How many pairs of paired appendages has a 

 crawfish ? If each pair of appendages indicates a seg- 

 ment, how many segments in the body of a crawfish ? Is 

 any segment of the body without a pair of appendages? 



16. Put a piece of the shell into strong vinegar or 

 other acid. If bubbles arise, it indicates that the 

 shell is composed in part of lime. 



The Internal Structure. 



1. Carefully remove the carapace and pin the 

 crawfish, back upward, under water. 



2. Find the heart. Note its shape and position. 

 Draw. 



3. Find one artery leading backward, and three 

 leading forward. To what organs do the forward 

 arteries go? 



4. Find holes, like slits, in the heart. How many? 



5. Find the stomach. Trace the passage from the 

 mouth to the stomach. Is it behind or in front of the 

 heart? Is it in the head or in the thorax? 



6. Slit open the stomach. Examine the inside. 

 Find three teeth, which constitute the principal part 

 of the grinding apjaaratus. Draw. 



7. Trace the intestine to its outlet. 



8. Distinguish other organs : liver, green gland, 

 and reproductive organs. 



9. See the muscles in the abdomen which bend 

 and straighten it. Which muscle bends it, and which 

 muscle straightens it? The bending muscle is the 

 flexor, and the straightening muscle is the extensor. 

 Which muscle is the larger? 



10. Separate the two parts of the flexor muscle 

 and find the nerve cord in the floor of the abdomen. 



