THE EARTHWORM 31 



Draw one backward across the hand. What is the sen- 

 sation? What is this due to? How many rows of the 

 bristles are there? (Use the magnifier to determine this.) 



Place the worm on a piece of glass. Can it progress? 

 Why this difference in the progression of the earthworm 

 on a smooth and on a rough surface ? What are the bristles, 

 or setce, for? Determine which way the setae point. 



Determine the sensitiveness of different parts of the body 

 by touching it in different places with the tooth pick. 



Note the blood tube along the middle of the back just 

 under the thin, transparent skin. Can pulsations in this 

 tube be seen ? If so, they would indicate a circulation of 

 the blood. Determine, if possible, which way the blood is 

 flowing in this tube. 



B. EXTERNAL FEATURES. Note the long, tapering, 

 cylindrical body. Is it one unbroken cylinder like a pencil ? 

 The rings are called segments. How many segments are 

 there in the body ? The end of the body that was usually 

 carried foremost while the worm was moving is the anterior 

 end, and the opposite end is the posterior end. The side of 

 the body held uppermost away from the ground is the 

 dorsal side, and the opposite side is the ventral side. Is 

 there any difference between the dorsal and ventral sides 

 in color and shape? If a cross section of the body were 

 made, would it be a perfect circle ? Which side would be 

 flat and which would be round ? 



Note that if the worm were cut in half lengthwise the 

 right half would be similar to the left half. This two- 

 sided likeness is known as bilateral symmetry, and the earth- 

 worm is a bilaterally symmetrical animal. 



Note the thickened, or swollen ring, the clitellum, near 

 the anterior end. Does it embrace more than one segment ? 



