SOME REACTIONS AMONG ANIMALS 167 



4. In how many directions will the entire leg move ? 



Conclusions. 1. Compare the movements of your arm and the joints 

 to accomplish these movements with the possible movements of a cheli- 

 ped. Which has the better arrangement to secure a great range of 

 movement ? 



2. Explain your answer in detail. 



c. The Gills 



Observations. 1. Use a specimen which has had the sides of the 

 carapace covering the gills removed. What is the general appearance of 

 the gills ? 



2. Move a cheliped. Is there a corresponding movement of any of the 

 gills? 



3. Look in the anterior part of the gill chamber and find the ' * paddles ' ' 

 which bale the water through the gill chamber. How are they shaped ? 



4. Examine the lower edge of the carapace that covers the gills. What 

 do you see ? 



Conclusions. 1. Is there any advantage in the attachment of gills to 

 the walking legs or mouth parts ? If so, what ? 



2. How are the gill "paddles " adapted to their function? 



3. Compare the method of getting oxygen in the sponge and crayfish. 

 Is there any similarity ? What ? 



4. How is the water strained as it enters the gill chamber? 



Drawings 



1 . Side view of a crayfish, natural size, with carapace removed from 

 one side to show gills. 



Label : cephalothorax, rostrum, abdomen, eye stalk, antennae, walking 

 legs, chelipeds, gills. 



2. Eye and stalk, and a few of the facets as seen through the microscope. 



3. Walking leg with gill attached. 



4. Drawing to show structure of abdomen and tail fin. 



Questions 



1. Crayfish are sold in large numbers in the markets of New York and 

 other large cities. Where do they come from ? 



2. How can they be shipped long distances without water ? 



3. When is the breeding season of the crayfish ? Of the lobster ? 



4. What other Crustacea are of economic importance ? In what ways ? 



5. Where are shrimps and lobsters obtained ? 



