LUMBRICUS. 87 



tives. The ganglia of each somite, and the cords that connect 

 those of adjacent somites, are fused so that the original paired 

 condition is not very apparent. How far does the ventral chain 

 of ganglia extend? Where do nerves leave it? 



Draw the nervous system into the figure that shows the repro- 

 ductive system. 



Notice the sacs that inclose the setae and indicate them in 

 the above figure. 



Examine prepared serial microscopic sections. 1 



1. The cuticle will probably be absent in most sections, in 

 which case the outer covering will be the cellular hypodermis or 

 skin. How many cells thick is this layer? Look for the gland 

 cells that keep the living worm moist. Do you know how the 

 cuticle is formed? 



2. Beneath the hypodermis is the circular muscle layer, which 

 is followed by the longitudinal muscle layer. The fibers of the 

 latter are arranged in conspicuous bundles. Lining the body- 

 wall is the thin peritoneal layer. Do you understand the func- 

 tion of each of these layers? How is the body elongated? 



3. Find the setae and determine where they are placed, how 

 many are in each group, how many groups there are, how they 

 pierce the body-wall, and what muscles are attached to them. 

 Why are setse not in every section? 



4. The alimentary canal consists of a lining epithelium, 

 followed by connective tissue and muscle, and, on its outer wall, 

 peritoneal cells, which in the region of the stomach-intestine 

 are large, very numerous, and are known as the chloragog cells. 



5. Lying in the mid-ventral line, beneath the alimentary 

 canal and close to the body-wall, is the ventral nerve cord. 

 Examine its structure. See if any of the sections show nerves 

 leaving it. 



1 Small worms should be kept in a dish and fed on clean moistened 

 filter-paper, which they will eat readily, until the alimentary canal is free 

 from grit, before they are preserved for sectioning. It is well to narcotize 

 them by placing them in a small quantity of water and adding a little 

 alcohol from time to time (never enough to make the worms squirm vio- 

 lently) until they cease to move. They may then be killed with subli- 

 mate acetic or other killing agent and treated in the usual manner. 



