INTERNAL ANATOMY OF THE CRAYFISH 293 



the structure as observed. Remove the stomach, open and note (d) the 

 large anterior cardiac portion which receives food from the esophagus 

 and which contains (e) the gastric mill with (/) chitinous denticles for 

 grinding. Note (g) the strainer between the cardiac and pyloric por- 

 tions. 



4. Examine the large segmentally arranged extensor muscles in the 

 abdomen and also the flexor muscles which run from the abdomen to 

 their anterior attachment on the right and left sides of the ventral wall 

 of the thorax. Understand the actions of these two sets of muscles. 



5. Separate the extensor muscles in the median line and locate the 

 underlying ventral nerve cord. Trace the latter throughout the length 

 of the abdomen noting the ganglia and the branches given off to the 

 adjacent muscles. Next trace the nerve cord anteriorly through the 

 thorax and to the anterior end of the body. In the thorax the cord lies 

 concealed under the muscle tissue and a series of chitinous plates. Care- 

 fully dissect and expose the nerve cord throughout the entire length of 

 the body. Note the double character of the cord. This is well-shown in 

 the posterior part of the thorax, where it is pierced by the sternal artery, 

 and also in the head, where the nerve cord passes around either side of 

 the esophagus and then unites again to form the important cerebral 

 ganglion, from which nerves are given off to the eyes, antennules, and 

 antennae. Find the sac-like green glands, which function in excretion, 

 lying near each eye at the base of the antenna, through which they open. 



