CRAYFISHES AND THEIR RELATIVES 153 



out and double up the parts of the abdomen ; of 

 how many segments is it composed ? 



11. (Optional.) The large appendages (large swimmer -els) and 

 the last segment of the abdomen taken together are 

 called the tail fin. Make a sketch (X 2) of the abdo- 

 men and the large swimmerets. Label: first 

 segment, last segment, large swimmerets, tail fin. 



D. Adaptations for breathing. 



To the Teacher. Prepare some preserved cray- 

 fishes in the following manner: Insert the point of 

 the scissors beneath the posterior margin of the cape 

 that covers the cephalothorax and halfway between 

 the middle line of the dorsal surface and the lower 

 margin of the cape ; cut forward to the front end of 

 the cape and remove the piece of shell. 



1. Immerse in water a crayfish prepared as directed 



above. Examine and describe the structures that 

 you find above the legs on the side where the 

 cape has been partially removed. These struc- 

 tures are the special breathing organs of the 

 crayfish. They are known as gills. 



2. Push the gills to one side and find the soft body 



wall. Higher up find the line of attachment 

 between the shell and the body wall. You 

 will see that the gills are not inside the body, 

 but in a space between the body of the 

 animal and its shell. This space is called the 

 gill chamber. 



a. In what region of the crayfish are the gill chambers 



found? 



b. What forms the outer wall of each gill chamber? 



What forms the inner wall ? 



c. Lift up the cape on the opposite side of the animal ; 



state where it is free from the body wall. 



3. Examine the gills on a leg that has been removed 



from the thorax and floated on water and note 



