44 STUDIES OF ANIMAL TYPES 



Note the number of segments in the abdomen. How 

 many appendages does the fourth segment, for instance, 

 bear ? What is the general shape of this segment, and on 

 what part of it are the appendages borne? The convex 

 piece on the dorsal side is called the tergum. The piece on 

 each side that slopes like the side of a roof is called the 

 pleurum. The transverse piece on the ventral side be- 

 tween the two appendages is called the sternum. How are 

 the segments of the abdomen connected with each other? 

 Why? 



The appendages of the crayfish. Now look at the end of 

 the abdomen and note that it ends in five flaplike bodies, 

 each fringed around the end. The broad middle flap is the 

 last segment of the abdomen, and is known as the telson. 

 The two flaps on each side of the telson are the greatly 

 modified appendages of the next abdominal segment an- 

 terior to the telson. These flaps are in pairs, and, together 

 with the telson, form the tail fin of the crayfish. 



The crayfish has nineteen pairs of appendages. Note that 

 the last segment, the telson, bears no appendages. The next 

 segment, the sixth, bears the two pairs of fringed flaps that 

 have already been spoken of as forming a part of the tail 

 fin. The next five abdominal segments bear each, one pair 

 of appendages, which are called the swimmerets. These 

 appendages are all alike except those on the first and second 

 segments, which are reduced and smaller, or perhaps lack- 

 ing entirely in the female. 



Note that each appendage consists of a short, twice- 

 segmented stalk, bearing two narrow, flat, fringed filaments, 

 like a two-tined fork. 



The next five pairs of appendages are the legs attached 

 to the thorax. Note that the fourth and fifth pairs end 



