THE CRAYFISH 45 



with one short claw, while the other three pairs end in 

 double claws, which are very large on the first pair. How 

 many segments in each leg ? 



After having examined the appendages as directed, begin 

 at the most posterior one, and remove those of the abdomen 

 from the right side, being careful to keep them in their 

 proper order. When the legs are reached, before removing 

 them, expose the gills. To do this begin at the posterior 

 end of the carapace, and cut through it with a pair of 

 scissors, along the middle, until the line separating the 

 head and thorax is reached. Then follow this line to its 

 anterior end. Break off the piece of the carapace so cut, 

 and the gills will be exposed. Note their feathery ap- 

 pearance. Note that they lie in a chamber completely 

 hidden from above by the carapace. Do they fill this 

 chamber? Where does the water get into the chamber? 

 Note that the gills are arranged in layers, or series. One 

 layer, or series, is attached to the bases of the legs while 

 the other series is attached to a thin, transparent mem- 

 brane. Now remove each leg with its gill and lay them in 

 order after the other detached appendages. The thorax 

 has three more pairs of appendages immediately in front 

 of the large pair of legs. These three pairs entirely cover 

 the mouth. Push them to one side and note the mouth 

 hidden by them. These appendages are called the foot- 

 jaws, or maxillipeds. They are used to aid in grinding the 

 food. 



Remove the posterior maxilliped. Note that it, like a 

 swimmeret, has a twice segmented stalk with two branches, 

 but, unlike the swimmeret, it bears a gill. Remove the 

 next maxilliped. Has it all of the parts of the first one? 

 What is the difference in the size of the two detached maxil- 



