164 SOME REACTIONS AMONG ANIMALS 



2. Note the feelers. The long ones are called antennae, the short ones 

 antennules. How are they arranged when the crayfish is at rest ? When 

 disturbed ? When crawling ? 



3. Wrap a bit of cotton on a toothpick or splinter and dip in ammonia. 

 Place this near various parts of the crayfish, especially the mouth parts and 

 feelers, but do not touch the crayfish with it. Does the craytish respond? 



4. Move the point of a pencil slowly toward the front of one of its 

 eyes. How does it respond ? 



5. How great is its range of vision ? That is, through what part of a 

 circle can a crayfish see without moving its body ? Compare this with 

 your own range of vision. 



b*. Place a cover over one half of tray. Where does the crayfish place 

 itself with respect to light or darkness ? 



Conclusions. 1. Name in order the parts of a crayfish that appear 

 the most sensitive. 



2. What seems to be the function of the antennae and antennules ? 



3. Where do you think the sense of smell is located ? 



4. What advantage has the crayfish gained by having its eyes mounted 

 on movable stalks ? 



5. What conclusions can you draw as to its sensitiveness to light ? 



6. Compare the sensibility of the crayfish with that of the worm. 

 Which do you consider has the keenest senses ? 



7. Compare likewise with a cat or dog. Which is the better organized 

 animal ? 



8. What light does the sensibility or reactive power of the crayfish 

 throw upon its position among animal forms ? 



9. What do you think is the real reason for the presence of sense 

 organs ? 



d. Feeding 



Note. A crayfish in captivity will not readily respond to offers of food, 

 but with patience some results may be obtained. 



Method. By means of a dissecting needle or wire, convey a piece of 

 beef or liver towards the mouth of the crayfish, and finally loosen it near 

 the appendages there. Note the reaction. (Beef juice may be used in 

 place of meat.) 



Observations. 1. What movements of the mouth parts do you see ? 



2. Does it make any use of its chelipeds ? 



Conclusions. 1. What appendages do you judge are adapted for seiz- 

 ing food ? 



2. What appendages may guide the food like fingers ? 



3. What appendages may cut the food or bite it ? The jaws on each 



