Lessons 13–15 · 3 lessons · O. Mavlonov. Biology (Zoology), Grade 7. Revised and expanded fourth edition. “O‘zbekiston milliy ensiklopediyasi” State Scientific Publishing House, Tashkent, 2017
15
Lab 2. Earthworm: external structure, movement and reflexes
Textbook: p. 52
GoalLearn how to observe the earthworm’s external structure, movement and reflexes safely in the lab.
New words
magnifying glass · lupadorsal and ventral sides · orqa va qorin tomonbristles (setae) · tuklarobservation · kuzatish
Explanation
The lab is done under the teacher’s supervision with a live worm in a tray of damp soil. First use a lens to find the front (cone-shaped, thin) and rear (blunter) ends, the mouth and the anus; then count the rings and find the girdle. The dorsal side is darker and rounded, the ventral side paler and flatter. On rough paper you can hear the bristles rustle as the worm moves; on smooth glass it moves with difficulty because the bristles find no grip. With the tip of a soft stick lightly touch different parts of the body and watch the response (the body contracting), but do not hurt the worm or prick it with a pin. Measure its length stretched and contracted and draw it. Afterwards return the worm unharmed to the soil.
Worked examples
Notebook entry: “Worm length stretched 10 cm, contracted 6 cm; more than 100 rings; girdle in the front part.” Precise notes like this prove the observation.
On glass the worm slips, while on paper its bristles catch and it moves forward. So bristles give support for movement.
Class activity
Work in pairs: one observes the worm, the other records results in a table (length, rings, reaction). Safety: never prick or squash the worm; wash your hands with soap after touching soil and worms; return the worm to the soil at the end.
Practice
1
How do you tell the worm’s front end from the rear?
The front end is thin, cone-shaped and near the girdle; the rear is blunter.
2
Why does the worm move with difficulty on glass?
Its bristles find no grip on a smooth surface.
3
What do responses to light or touch show?
The worm senses the stimulus through its skin and contracts or moves to the dark – these are reflexes.
4
Why must the worm be returned to the soil at the end?
It is a living creature; without damp soil it dries out and dies. Careful treatment of nature is needed.