Observation: Anatomy of
External Anatomy
1. Examine a live or preserved specimen of an earthworm. Locate the small projection that sticks
out over the mouth. Has cephalization occurred?
Explain your answer.
2. Count the total number of segments, beginning at the anterior end. The sperm duct openings are on
segment 15 (Fig. 24.7). The enlarged section around a short length of the body is the clitellum. The
clitellum secretes mucus that holds the worms together during mating. It also functions as a cocoon,
in which fertilized eggs hatch and young worms develop. The anus is located on the worm's terminal
segment.
3. Lightly pass your fingers over the earthworm's ventral and lateral sides. Do you feel the setae?
Earthworms insert these slender bristles into the soil. Setae, along with circular and longitudinal muscles,
enable the worm to locomote. Explain the action.
Figure 24.7 External anatomy of an earthworm.
In this drawing, the segments are numbered.
