150 THE BIOLOGY OF THE ANMAL 



body does it lie? What separates the branchial 

 cavity from the body-cavity ? At which point does 

 it commnnicate with the exterior ? Look for the 

 cervical canal, in which lies the scaphognathite. 

 Does the branchial cavity contain any organs 

 besides gills ? Notice that the gills may be divid- 

 ed into three groups: (1) podobranchise, those 

 attached to the exopodites of the appendages; 

 (2) arthrobranchise, attached to the articular 

 membranes ; and (3) pleurobranchise, attached 

 to the inner wall of the branchial chamber. 



Eeview all of the appendages and note those 

 on which you found gills. How many arthro- 

 branchise do you find? Do you ever find two 

 on the same membrane? If so, on which? 

 "What position have the arthrobranchise with 

 reference to those attached to the appendages ? 

 How many pleurobranchiEe are there? What 

 is their position ? In which group are the gills 

 largest ? Do any of them branch ? Do all have 

 the same general shape ? Are the giUs within 

 the body? Kemove in succession one of the 

 largest gills of each group, place it in a dish of 

 water, and examine its structure. Is the gill stiff 

 or flexible ? Do all have the same structure ? 

 If notj which are the most complicated ? Notice 

 the finest divisions, the branchial filaments, of 

 the gill. In the crayfish, note that some of the 

 pleurobranchise are rudimentary. Can you give 

 reasons for calling them " rudimentary " ? 

 Draw a gill of each kind. 



K The exoskeleton or cuticle.— What is its nature? 

 Color? Thickness? Is it the same on the ap- 



