THE ARTHROPODS 361 



appendages. Examine all the appendages, and sketch those from 

 the fourth and sixth segments. 



Study a living crayfish with reference to the uses of the appen- 

 dages of the abdomen. 



Gills. Underneath the sides of that part of the carapace cover- 

 ing the thorax are the gills or organs of respiration. Raise with 

 forceps the ventral edge of the carapace on one side and notice that 

 a number of feather-like structures fill the underlying cavity. They 

 are the gills, and the cavity is the gill-chamber, while the part of the 

 carapace which covers it is the gill-cover. With strong scissors, cut 

 away the gill-cover on one side, so as to expose the gills. Cover the 

 animal with water and notice the arrangement of the gills. Move 

 some of the appendages and notice that some gills are attached to 

 their bases, while others are attached to the inner wall of the gill- 

 chamber, which is really the body-wall. Note that the gills are 

 outside of the body-wall and are therefore external structures. 

 The gill-cover is simply a fold of the body-wall. It is difficult to 

 count gills in position, and this will be done later after removing the 

 appendages. Make a diagram of the gill-chamber showing arrange- 

 ment of the gills. In the extreme anterior end of the gill-chamber 

 may be seen a "paddle" or "scoop" which is attached to one of the 

 appendages of the mouth. See Fig. 116. 



(D) Observe the movements of this "scoop" in a living animal 

 from which a small piece of the gill-cover has been cut (a painless 

 operation). 



Observe the direction of the currents as shown by powdered 

 carmine or indigo placed in the water near the bases of the posterior 

 walking legs. In your diagram of the gill-chamber insert arrows to 

 show direction of the currents. 



Appendages of the Cephalothorax. Observe the locomotion of a 

 crayfish. How many pairs of legs are used in walking ? 



(D or L) On the side of the body from which the carapace was 

 cut remove the appendages, beginning with the most posterior 

 walking leg, carefully cutting with strong scissors the muscular 

 attachments to the body. Take care to keep the gills in place and 

 the appendages in order. Examine the appendages in order of 

 removal, comparing with a mounted series in order to make sure 

 that your specimens are complete. Number the appendages, 

 beginning with the first antenna. What appendages bear gills? 

 Count the gills which remain attached to the body-wall. What 

 is the total number? 



Do you find the appendages of the cephalothorax arranged in 



