166 SOME REACTIONS AMONG ANIMALS 



6. Drop a bit of the shell in dilute hydrochloric acid and observe the 

 result. When this acid is placed on a substance and it effervesces or 

 forms bubbles of gas, you may expect the substance to contain lime. 



7. The shell over the dorsal part of the cephalothorax is called the 

 carapace. It extends forward into a beak, called the rostrum. Find 

 these parts. 



8. Is the carapace continuous with the ventral portion of the shell ? 



9. Lift the edge of the carapace or cut it away from one side, and 

 find the gills under the shell. The space inclosed by the overlapping of 

 the shell is called the gill chamber. 



10. How are the segments of the abdomen united with one another ? 



11. Has the tail segment, or telson, a pair of appendages? If not, 

 how are they related to the telson ? 



12. Where is the skeleton of a crayfish located ? 

 Conclusions. 1. What is the most flexible portion of the body ? 



2. How is the abdomen adapted for flexibility ? 



3. What is the office of lime in a shell ? 



4. How is the caudal fin adapted for swimming? 



5. Are the gills external or internal structures ? Explain your answer. 



6. What differences in habitat may be expected of the crayfish as com- 

 pared with a worm, because of its well-developed skeleton ? What dif- 

 ferences in habits ? 



7. How can you tell the presence of lime in a shell or skeleton ? 



2. APPENDAGES 



a. The Eye 



Observations. 1. Note the eye stalk, and the eye at its tip. (Use 

 hand lens.) Can it be moved in all directions ? 



2. Note the thick shell covering the cornea. Study a small bit of the 

 cornea mounted on a slide in glycerine, using low power. 



3. Note the small areas (facets} of which the cornea is composed. 

 These mark the outer ends of the single eyes. Since the eye of a crayfish 

 is composed of many such single eyes, it is called a compound eye. 



b. The Legs 



Observations. 1. How many walking legs do you find? How many 



2. To what region are the walking legs attached ? 



3. Bend each joint of a cheliped. In how many directions will a, single 

 joint of a cheliped move ? 



