THE CRAYFISH. 25 



ing how the segments are jointed together, and how 

 they move one upon another. 



Separate the third ring (counting from the front) 

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

 hold the cephalothorax and fore part of the abdomen 

 firmly between the thumb and forefinger of the left 

 hand, with the posterior end of the abdomen projecting 

 toward the right hand ; then, grasping the dissecting 

 needle firmly 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 

 without breaking either ; with scissors cut the mem- 

 brane between the under sides of the rings, and en- 

 tirely separate them. In like manner, detach the third 

 segment from the second. The ring has these parts: — 



a. The upper part, the tergite. 



b. The under part, the sternite. 



c. The side piece, the pleurite (projecting down- 

 ward). 



d. Two appendages, the swimmerets. 



Observe that each swimmeret has a main stalk and 

 two branches ; examine these appendages thoroughly. 

 Lay the ring on its front side, make the branches of 

 the swimmerets diverge enough to appear distinct, 

 and make a drawing of the whole ring as seen from 

 behind. 



Compare the other segments of the abdomen with 

 the third. 



In the mate the appendages of the first and second 

 rings are larger than in the female. Other organs as 



