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. 
