40 INVERTEBRATE ZOOLOGY 



just ventral to the stomach, taking care not to injure the brain, 

 which lies in front of the stomach, or the two slender nerve- 

 connectives, which lie on either side of the oesophagus. Sever 

 the rectum near the anus. Remove the entire digestive tract 

 from the body and place it in a pan of clean water. The liver 

 is so soft that it may not be possible to remove it entire. 

 Notice the boundary between the intestine and the somewhat 

 larger rectum. In the crayfish the rectum is much longer than 

 the intestine; in the lobster the opposite is true. In the lob- 

 ster notice the blind-gut or appendix which joins the rectum near 

 its anterior end. 



Exercise 23. Make a diagrammatic sketch of the digestive tract. 



Cut open the stomach by a ventral incision and wash it out. 

 Observe its chitinous lining and the dark brown chitinous teeth. 

 This chitinous lining is a continuation of the cuticula which 

 covers the external surface of the body and is moulted with 

 the cuticula. During certain parts of the year a pair of large 

 calcareous bodies called gastroliths are imbedded in the lining of 

 the stomach. They remain in the stomach after the moulting 

 of the cuticula and furnish lime for the new cuticula, which at 

 once grows rapidly. 



Exercise 24. Make a sketch of the inner surface of the stomach 

 showing the teeth. 



The excretory system. Notice in the extreme forward end of 

 the body-cavity, just in front of and below the stomach, a pair 

 of pale greenish bodies. These are the kidneys or green glands. 

 Each one is made up of two portions, the smaller glandular por- 

 tion, next to the body-wall, and the larger saccular portion, or 

 urinary bladder, next the stomach. From the latter the ureter 

 leads to the external openings which have already been noted. 



Exercise 25. Draw a view of the forward end of the body- 

 cavity showing the kidneys as they lie in position. 



