68 ELEMENTARY ZOOLOGY. 



each seem to be specially suited to the work which it 

 does. Find what advantages accrue from having the 

 different types where they are rather than elsewhere. 



Are the four walking appendage ^homologous (see p. 64) 

 with each other? Are the large nippers (chela) homol- 

 ogous with these? Evidences? Are the small abdominal 

 swimmer ets homologous with one another? With the 

 walking appendages? What observations and argu- 

 ments can you advance in favor of the view that all these 

 paired appendages are homologous? What against such 

 an interpretation? What is your real conviction in the 

 matter? What arguments can you advance to support 

 (or to combat) a view that a pair of appendages indicates 

 a segment of the body, no matter where they are found ? 



Remove the arch of the carapace on one side of the 

 thorax and expose the gills. Cover the animal with 

 water and float the gills out so that you can study them 

 in place. Are they related in any way to the appendages? 

 In what way? With how many appendages are gills 

 found? How many gills to each appendage? What 

 is the structure of the gill? Examine under the lens 

 and draw one gill. 



Remove the appendages in front of the chela, one at a 

 time, being careful to get the entire appendage. How 

 many kinds, and how many of each kind, do you find? 

 Try to determine their possible use. Make a careful 

 drawing of one of each kind of appendages, beginning 

 with the abdominal ones and going forward. Study the 

 tail. Is it a segment with appendages? Has the last 

 segment of the body any appendages? 



Classify the appendages of the crayfish according to 

 the following scheme. Carry out your analysis until 

 only those that are clearly alike are found together. 



