AN EARTHWORM 75 



systems, the circular and the longitudinal muscles. The former 

 are a narrow band just beneath the hypodermis. The latter are 

 much more extensive and project into the body-cavity ; they are 

 arranged in groups and will be seen of course in cross section. 

 Near the center of the body-cavity note the large alimentary canal. 

 If the section be in the region of the stomach-intestine, note the 

 longitudinal fold in the dorsal intestinal wall which very largely 

 increases its surface. Observe the structure of the alimentary 

 canal ; its cavity or lumen is bounded by a thick mucous mem- 

 brane consisting of a single layer of very long, slender cells, 

 around which are two muscle layers, an inner circular and an 

 outer, very thin, longitudinal layer. Surrounding the muscle 

 layers and also forming a thick fold over the dorsal and lateral 

 intestinal surfaces are the pear-shaped chloragogue cells. Observe 

 the dorsal and the ventral blood vessels, and also the commissural 

 blood vessels, if any are in the section. Study carefully the 

 nervous system. Note the muscular sheath which surrounds the 

 nerve cord, and imbedded in it the subneural and the two latero- 

 neural blood vessels. Note the double nature of the nerve. Note 

 the large pear-shaped nerve cells, and the nerve fibers, also the 

 three large bodies in the dorsal portion of the ganglion. These 

 latter are called the giant fibers. Do lateral nerves join the 

 ganglion ? If so, trace their fibers into it. Also trace their 

 fibers away from the ganglion and see where they go. Exam- 

 ine carefully the peritoneum. This is a layer of cells which lines 

 the body-cavity and bounds all the organs in it. 



Exercise 8. Draw the cross section and carefully label all the 

 organs. 



