34 Practical Zoology. 



during the day or during the night? How does the crayfish build 

 the chimney? How deep are the holes? What are they for? 

 Do all kinds of crayfishes dig holes? In what part of the hole 

 does the crayfish stay? Are the holes used equally at all seasons? 



Molted Shells. If you find what appears to be a dead cray- 

 fish, examine it carefully to see whether it is really a dead animal 

 or only the cast-off shell. Do you find any dead ones? 



Enemies of the Crayfish. Have you seen any animal eating or 

 attacking a crayfish? Or any evidences of such action? 



HOME OBSERVATIONS. 



Walking. If one has no aquarium, a dish pan or homemade 

 trough serves very well. Watch a crayfish crawling in the water. 

 What appendages are used ? Can it walk in other directions than 

 head foremost? Are the legs moved in regular order? Place a 

 crayfish on the floor. Does it walk equally well in water and out ? 

 Why should there be a difference ? 



Swimming. Frighten a crayfish by thrusting a stick at it to 

 see how it swims. Study closely the parts used and their action. 

 Suppose a crayfish could propel itself rapidly forward, how would 

 the resistance compare with the resistance it meets while going 

 backward? Note closely the condition and position of the tail 

 fin while making the stroke and while darting through the water 

 between strokes. Observe all the points of structure that aid the 

 efficiency of the stroke? What is true of the amount of resistance 

 in the recover stroke? With the thumb and finger, take a cray- 

 fish just back of the big pinchers and hold it with the head up, so 

 that the tail fin is covered with water ; if it is now excited, the 

 effectiveness of the tail fin will be well demonstrated. 



Mode of Defense. With a stick or pencil, make motions at a 

 crayfish to see how it defends itself. Allow it to grasp a pencil to 

 show the strength of its grip. Does a crayfish prefer to fight, or 

 would it rather avoid an attack? In an aquarium does, a crayfish 



