DISSECTION OF CRAYFISH. 161 



wards and see the oesophagus or tube leading to it from 

 the mouth. Tip it forwards and find the intestine. Can 

 you find the ducts leading from the liver to the intestine ? 



Draw the viscera, etc., as far as made out, adding the 

 intestine later. 



Cut away heart, liver, reproductive organs, and trace 

 the intestine to the vent. Is it the same size throughout ? 



Take out the stomach, being very careful not to injure 

 other structures when cutting the oasophagus. Open the 

 stomach and find the teeth; how many ? Try to see how 

 the teeth grind the food. 



In the front part of the body, close to the antennae, find 

 the green glands (paired). Their openings have already 

 been found. They are excretory in function (kidneys). 



Cut away the (white) muscles in the abdomen, being 

 careful as you approach the floor, and expose the hinder 

 part of the central nervous system (ventral cord). How 

 are the enlargements (ganglia) arranged with reference to 

 the segments of the abdomen ? Are the nerves given off 

 from the ganglia, or from the cord (commissure) connecting 

 them ? Trace the ventral cord forward into the cephalo- 

 thorax, carefully breaking away the hard parts which cover 

 it, and follow it forward to the brain, in front of the 

 mouth. How many ganglia do you find in the cephalo- 

 thorax ? Do any show signs of being double ? Is the 

 commissural cord single or double in this region ? Is there 

 a ring of the nervous system around the oesophagus ? Can 

 any of the nervous system be said to be above, or any 

 below, the alimentary canal ? From what part do the 

 nerves to the antennae and eyes arise ? 



Draw the nervous system from above. 



