THE CRAYFISH' 95 



(2) The united head and thorax are called the cephlo- 

 thorax. 



(3) The continuous covering of the two is the carapace; 

 above and between the eyes is the rostrum. 



(4) The hinder, flexible part is the abdomen. Count its 

 rings, or segments. Bend (flex) the abdomen, and straight- 

 en it repeatedly, observing how the segments are joined 

 together and how they move upon one another. 



(5) Separate the third abdominal ring (counting from 

 the front) from the rings in front of and behind it. To do 

 this hold the cephalothorax and fore part firmly between 

 the thumb and fore finger of the left hand with the 

 posterior end of the abdomen projecting toward the right 

 hand; then grasping the needle with the right thumb and 

 forefinger, thrust the point of the needle obliquely forward 

 between the third and fourth segments and work it right 

 and left, severing all connection between them; with the 

 scissors cut the membrane between the under sides of the 

 rings and entirely separate thern. 



(6) In like manner detach the third segment from the 

 second. Observe the following parts : 



a. The upper part — the tergiie. 



b. The under part — the sternite. 



c. The side piece — the pleurite. 



d. Two appendages — the swimmerets. 



Find that each swimmeret consists of a main stalk and 

 two branches. Examine these branches thoroughly. Lay 

 the ring on its front side and make a drawing of the com- 

 plete ring. Compare the other rings of the abdomen with 

 the third. Draw ring with swimmeret (X2). 



In the male crayfish the swimmerets on the first abdom- 



