Fig. 69.—An Earthworm.
External Features.—Is the body bilateral? Is there a dorsal and a ventral surface? Can you show this by a test with live worm? Do you know of an animal with dorsal and ventral surface, but not bilateral?
Can you make out a head? A head end? A neck? Touch the head and test whether it can be made to crawl backwards. Which end is more tapering? Is the mouth at the tip of the head end or on the upper or lower surface? How is the vent situated? Its shape? As the worm lies on a horizontal surface, is the body anywhere flattened? Are there any very distinct divisions in the body? Do you see any eyes?
Experiment to find whether the worm is sensitive (1) to touch, (2) to light, (3) to strong odours, (4) to irritating liquids. Does it show a sense of taste? The experiments should show whether it avoids or seeks a bright light, as of a window; also whether any parts of the body are especially sensitive to touch, or all equally sensitive. What effect when a bright light is brought suddenly near it at night?
Is red blood visible through the skin? Can you notice any pulsations in a vessel along the back? Do all earthworms have the same number of divisions or rings? Compare the size of the rings or segments. Can it crawl faster on glass or on paper?
Fig. 70.—Mouth and Setæ.
A magnifying glass will show on most species tiny bristle-like projections called setæ. How are the setæ arranged? (d, Fig. [70].) How many on one ring of the worm? How do they point? Does the worm feel smoother when it is pulled forward or backward between the fingers? Why? Are setæ on the lower surface? Upper surface? The sides? What is the use of the setæ? Are they useful below ground? Does the worm move at a uniform rate? What change in form occurs as the front part of the body is pushed forward? As the hinder part is pulled onward? How far does it go at each movement? At certain seasons a broad band, or ring, appears, covering several segments and making them seem enlarged (Fig. [71]). This is the clitellum, or reproductive girdle. Is this girdle nearer the mouth or the tail?
