78 A GUIDE FOR THE STUDY OF ANIMALS 



moved and placed in order on a piece of paper upon which 

 a list of the appendages has been written. , 



10. How many pairs of appendages are there ? How 

 may they be grouped according to location ; how grouped 

 according to function ? How many pairs are there in each 

 group ? 



1 1 . What similarities of structure do you find in nearly 

 all of the appendages ? Assuming a swimmeret of the 

 third, fourth, or fifth somite to be the least changed from 

 the primitive type, what changes were necessary to make 

 the sixth swimmeret ; the third maxilliped ; the walking 

 legs ; the antennae ; the antennules ? 



12. Remove the part of the carapace which covers a. 

 gill chamber. What are the boundaries of the chamber .■' 

 Where does it open to the water .? 



13. Describe the appearance and the texture of a gill. 

 How are the gills kept moist when the crayfish is in 

 water ; when it is on land ? Why should they be kept 

 moist ? 



14. Would you class the gills as external structures or 

 as internal ? Why do you think so ? To what are they 

 attached.' How are the gills affected by the motion of 

 the legs ? 



15. What work goes on in the gills ? How is the supply of oxygen 

 renewed ? In this connection, try a live crayfish, kept quiet in water 

 just about deep enough to cover it. Float bits of paper near it or 

 carefully place a drop of ink in the water near it. By some such method 

 currents of water may usually be shown, and their direction determined. 

 Consider also the habitual motions of mouth parts and swimmerets, the 

 bubbles sometimes seen when a crayfish is dropped into water and the 

 habit crajrfishes have of lying on one side, close to the surface of the 

 water. 



