A CRAYFISH OR A LOBSTER 31 



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 lobster 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 molted with the 

 cuticula. During certain parts of the year a pair of large, cal- 

 careous bodies called gastroliths are embedded in the lining of the 

 stomach. They remain in the stomach after the molting 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 to 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. 



The nervous system consists of a ventral double nerve cord 

 lying in the midventral line in the body cavity and extending 

 the length of the animal, with paired ganglia at intervals, also of 

 a brain situated just back of the eyes, which is united with the 

 ventral nerve by two nerve connectives, passing one on each side 

 of the oesophagus. The ventral ganglia have typically a meta- 

 meric significance, but many of the somites have lost their ganglia, 



