96 • ARTHROPODS 



inal segments are highly developed; in the female they are 

 degenerated and almost absent. Examine a number of 

 specimens to illustrate this. 



(7) The last segment is called the telson. It forms 

 the middle flap of the cmidal, or tail fin. Has it swimmer- 

 ets? Are there swimmerets on the sixth segment? Com- 

 pare the flaps on the sides of the caudal fin with the swim- 

 merets of the other segments. If they have the same parts 

 and seem to have the same origin they are said to be hom- 

 ologous. Are all of the swimmerets homologous with those 

 of the third segment ? Draw caudal fin ( X i ) • 



(8) Are the appendages of the thorax borne upon rings? 

 After looking carefully for rings, observe the appendages. 

 Are they homologous with one another? 



(9) Remove one half of the carapace with scissors and 

 forceps. This exposes the gills. Observe the gills closely, 

 noting structure and place of attachment. Move the legs 

 back and forth and see if there is any indication of a con- 

 nection between the gills and the legs. 



(10) Remove a gill and study its structure under the 

 microscope. Make out a tnain stalk and numerous filaments. 

 Study from books or other sources, the method of circula- 

 tion in the gills and the process of oxygenation of the blood. 

 Make a drawing of gill ( X i ) , also a diagram illustrating 

 the method of purifying the blood. 



(11) Study the last thoracic appendage or hindmost 

 walking leg. Count its segments observing carefully the 

 peculiarities of each. Notice the number of joints and the 

 motions allowed by each. With the forceps seize the basal 

 segment and pull off the leg. 



