A CRAYFISH OR A LOBSTER 



29 



Write opposite the number of each somite, in your notebook, 

 first, the name of the appendage belonging to it, then indicate 

 what parts that appendage possesses by a " -f " and what parts 

 it lacks by a " " under the appropriate head, as is shown 

 above in the case of the sixth somite. 



The gills. Remove the left-hand branchiostegite. Place the 

 animal in water and study the gills on the left side. These 

 organs may occur on the eight thoracic somites, and on each 

 segment they may be attached either to the basal joint of the 

 leg, when they are called podobranchiae, to the flexible joint by 

 which the leg articulates with the body, when they are called 

 arthrobranchiae, or to the body-wall just above the leg, when they 

 are called pleurobranchiae. A single thoracic somite may bear 

 on each side four gills, a pleurobranch, two arthrobranchs, 



