XII.] THE FEESH-WATER CRAYFISH. 143 



by a duct on each side at the point where the 

 intestine joins the stomach. 

 /S. Tease out a bit of the liver in water; it is 

 made up of branched cascal tubes, which when 

 examined microscopically are seen to be lined 

 by a layer of cells {epithelium). 



d. Carefully remove the alimentary canal, cutting 

 the gullet through close to the stomach. 

 a. Open the latter under w^ter and make out in 

 it the constriction which divides it into an 

 anterior {cardiac) portion and a posterior 

 {pyloric), 

 p. The supporting bars and the hairs in the 

 stomach, and the calcifications of its lining 

 membrane. 



15. Now trace the sternal aiieiy (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 

 oif 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 abdominaMpart of the nervous chain 

 will be exposed. It lies immediately above the blood- 

 vessel and is not to be injured. 



16. The nervous system. c^f..., 



a. Find the supraoesophageal ganglion in_£cQnt of 



the gullet. 

 y8. The circumoesophageal commissures passing 



back from it. 



