Crustacea. 37 



2. The covering of the cephalothorax is the carapace. Run- 

 ning across the carapace is the cervical groove. The anterior 

 projection of the carapace is the rostrum. 



3. Bend (flex) the abdomen, and straighten (extend) it re- 

 peatedly, observing how the segments are jointed together, and 

 how they move one upon another. Count its rings or segments. 



4. Separate the third ring (counting from the front) from the 

 rings in front of and behind it. To do this hold the cephalo- 

 thorax and fore part of the abdomen by the thumb and fore 

 finger of the left hand, with the posterior end of the abdo- 

 men projecting toward the right hand; then, grasping the 

 dissecting needle firmly with the right thumb and forefinger, 

 thrust the point of a dissecting needle obliquely forward between 

 the third and fourth segments, and work it up and down, severing 

 all connection between them without breaking either ; with scissors 

 cut the membrane between the under sides of the rings, and 

 entirely separate them. In like manner detach the third segment 

 from the second. The ring has these parts : 



a. The upper part, the tergum. 



b. The under part, the sternum. 



c. The side piece, the pleurum (projecting downward). 



d. Two appendages, the swimmerets. (See Fig. 46.) 



5. Observe that each swimmeret has a main stalk, the protopod, 

 and two branches, an outer, or exopod, and an inner, or endopod ; 

 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 male the appendages of the first and second rings are 

 larger than those of the other segments and are specially modified. 

 In the female the swimmerets of the first and second abdominal 

 segments are smaller than the others. The abdomen of the female 

 is wider than that of the male, probably for the purpose of pro- 

 tecting the eggs and young, which are attached to the swimmerets. 



