xii.] THE FRESH-WATER CRAYFISH. 149 



d. Carefully remove the alimentary canal, cutting 

 the gullet through close to the stomach. 



a. Open the latter under water and make out in 

 it the constriction which divides it into an 

 anterior (cardiac) and a posterior (pyloric) 

 portion. 



/?. The supporting bars and the hairs in the 

 stomach, and the calcifications of its lining 

 membrane. 



15. Now trace the sternal artery (removing the ali- 

 mentary canal and the genital organs), until it enters 

 a passage (sternal canal) formed by ingrowths of the 

 exoskeleton near the ventral surface of the animal. 

 Just before entering this the sternal artery gives 

 off the inferior abdominal branch, which runs back 

 along the middle line of the abdomen immediately 

 inside the sterna of the somites. Trace this branch 

 back removing the muscles which cover it. By this 

 proceeding the abdominal part of the nervous chain 

 will be exposed. It lies immediately above the blood- 

 vessel and is not to be injured. , 



1 6. The nervous system. 



a. Find the sitpraozsophageal ganglion in front of 

 the gullet. 



/?. The circumozsophageal commissures passing 

 back from it. 



y. Follow back these commissures, cutting away 

 the hard parts (forming the roof of the sternal 

 canal) which come in the way; they lead to 

 a chain of six ganglia, lying along the floor of 

 the cephalothorax, and united by double cords 



