FOR WAGGONER’S BIOLOGY 
61 
rest of the glove. In the ectoderm you will observe nematocysts, 
ovoid structures which contain stinging threads. If a hydra is 
mounted with the cover glass resting directly upon it, a light tap 
on the cover glass will crush the animal. This will free many of the 
nematocysts. The threads in some will be discharged and can be 
examined. 
EXERCISE 65 
THE EARTHWORM 
I. The Living Earthworm. Live earthworms may be brought 
into the laboratory and kept in a box of the soil in which they were 
found. Let members of the class remove earthworms from the soil, 
noting any difficulty which may be encountered in the process. 
Place a specimen on white paper. Note the direction and manner 
of its movement. Which is the anterior end? Note the slender 
dark structure running lengthwise within the body. This shows 
through the dorsal surface and is the doTsol blood vessel. Note its 
contractions. Try to draw an earthworm backward over the paper. 
Compare the dorsal and ventral surfaces by rubbing lightly with 
the finger. Experiment to find out whether the animal is sensitive 
to sound, light, food, etc. Touch the anterior end. Touch the 
posterior end. Which is the more sensitive? 
II. External Structure. (Preserved specimens may be used, 
if desired.) Note the general shape of the body. How does its 
thickness compare with its length? The body is regularly divided 
into segments by grooves extending around it. At the anterior end 
is the protruding upper lip or prostomium. This is not a true seg¬ 
ment. Some distance back of the prostomium is a somewhat 
swollen region, the clitellum. (Glands in the clitellum secrete a 
cocoon which collects both the egg and the sperm cells. Within 
this structure fertilization and the development of the egg and the 
young earthworm occur.) In the anterior part of the body there 
are thirty-seven segments when the clitellum and all the segments 
anterior to it are counted. How many do you find posterior to the 
clitellum? The mouth is on the ventral surface just behind the 
