3 6 Practical Zoology. 



these experiments take care not to produce such vibrations of air, 

 water, or mud as might effect the sense of touch. 



Test the sense of smell by placing various odorous substances 

 near the crayfish, when in air as well as in water. Is any attention 

 paid to scents ? 



Recall any choice of food the crayfish has made? Is this 

 choice determined by a sense of taste ? Place various substances 

 that affect your sense of taste on the crayfish's food. Does it 

 make any difference in his choice? Place on the mouth organs 

 drops of various liquids that affect your taste. Is the crayfish 

 affected thereby? 



Molting. Sometimes when one has left a single crayfish in an 

 aquarium he is surprised to find two. The molted shell looks like 

 a complete crayfish. Watch a crayfish closely for this change. 

 Before the molt a crayfish is dull and quiet. What are the first 

 stages of the process, and in what order does he cast off the old 

 shell ? Feel of the newly emerged animal. For several days test 

 the hardness of the shell. How long does it take for a "soft- 

 shell " crayfish to become a " hard-shell " ? What makes the shell 

 hard? Drop a piece of the old shell in weak hydrochloric acid 

 (vinegar will serve). Hold a piece of shell in flame. Does it 

 burn? 



Development. Find a female crayfish with eggs. To what are 

 the eggs attached ? Watch till the eggs hatch out. How long 

 are the little crayfishes when first hatched ? Do they go free or 

 remain attached to the mother? If they separate from her, do 

 they return to her? Does she make effort to keep near them 

 or keep them near her? Does she feed them? What do they 

 eat at first? Do crayfishes ever eat each other? Do they kill 

 each other? 



EXTERNAL PARTS OF THE CRAYFISH. 



i. Note the two distinct parts of the body, (i) the anterior, 

 rigid part, the cephalothorax ; (2) the posterior, flexible part, 

 the abdomen. 



