152 ANIMAL BIOLOGY 



C, Adaptations for swimming. 



Place an active crayfish in a pan nearly 

 filled with water. Use the following means 

 to get it to swim : make a sudden movement 

 toward it with the forceps or pencil; if this 

 does not succeed, take hold of the animal near 

 the anterior end where you can press the large 

 pincers against the body. Do this quickly and 

 release the animal. This action may cause the 

 crayfish to swim in order to escape. If you 

 cannot get this crayfish to swim, try another. 



1. In what direction does the crayfish swim? 



2. State whether or not the legs are used in swimming. 



3. Watch the segments of the abdomen and the large 



appendages at the posterior end to determine 

 their action in swimming. 



a. Describe the direction of the movements of these 



parts. 



b. Are these movements made slowly or quickly? 



4. In what direction will the doubling under of the ab- 



domen tend to send the animal ? 



5. In what direction will the straightening out of the 



abdomen tend to send the animal? 



6. In what direction, therefore, must the crayfish strike 



the harder and quicker in order to swim back- 

 wards ? 



7. What difference is there in the shape of the ventral 



surface and the dorsal surface of the abdomen ? 



8. Which surface of the abdomen will enable the cray- 



fish to get the better hold upon the water? 



9. (Optional.) Straighten out and double up the segments of 



the abdomen, noting how the segments are con- 

 nected. Describe now all the adaptations of the 

 abdomen and its appendages for swimming? 

 10. (Optional.) The first segment of the abdomen (next to the 

 cephalothorax) fits under the cape ; the last is un- 

 like the others in shape, being quite flat. Straighten 



