THE CRAYFISH 35 



Cut the stomach open. Note an interior portion, the 

 cardiac chamber, and a smaller posterior portion, the pyloric 

 chamber. Examine its inner surface. What do you find 

 here ? This structure is called the gastric mill. Food, which 

 for the most part consists of any dead organic matter, is 

 chewed by the "stomach-teeth" into fine bits, and is then 

 passed into the pyloric chamber. It is here that the diges- 

 tive glands empty their secretion into the food. These glands 

 have the same office as have the liver and pancreas combined 

 in the toad, and so they are often called the hepato-pancreas. 

 AVhen the stomach has been removed there will be noted in 

 the anterior portion of the body paired, flattened bodies, 

 already mentioned, which connect with openings at the base 

 of each of the antennae by means of wide thin-walled sacs, 

 the ureters. These organs are the kidneys, or green glands. 

 Their office is similar to that of the kidneys in the toad, 

 namely, the elimination of waste from the body. 



Technical Note. — Carefully remove all of the alimentary canal, 

 digestive glands, and reproductive organs. This process will expose 

 the floor of the cephalothorax. Now cut away from either side the 

 horny floor or bridge at the bottom of the cephalothorax. If the 

 specimen has not already been immersed, place it in clear water for 

 further dissection. 



The foregoing dissection will expose the central nervous 

 system. It extends as a series of paired ganglia connected 

 by a double nerve-cord along the ventral median line from 

 the oesophagus to the last segment of the abdomen. From 

 what points do the lateral nerves arise? Anteriorly the 

 double nerve-cord divides, the two parts passing upward on 

 each side of the oesophagus, where they again meet to form 

 the supra-ossophageal ganglion or brain. Where do the 

 nerves run which rise from the brain? What is the differ- 

 ence between the position of the central nervous system in 

 the crayfish and in the toad? 



