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
14
Earthworm: internal structure
Textbook: pp. 48–51
GoalExplain the earthworm’s digestive, circulatory, respiratory and excretory systems and the importance of annelids.
New words
crop · jig‘ildonclosed circulatory system · yopiq qon aylanish sistemasicapillary, the finest blood vessel · kapillyarvermicompost, fertiliser made with worms · biogumus
Explanation
Digestive system: mouth, pharynx, gullet, crop, gizzard-like stomach, intestine and anus. The worm swallows decaying plant matter with soil and passes it through the gut. The circulatory system consists of dorsal and ventral blood vessels joined by ring vessels; 5–6 muscular ring vessels around the gullet act as “hearts”. Blood flows only inside vessels – a closed circulatory system, which first appeared in annelids. Blood flows forward along the dorsal vessel and backward along the ventral one. The worm breathes through its skin: oxygen passes into the blood in capillaries and carbon dioxide leaves. The excretory system is a pair of thin tubes in each segment with funnel-shaped, ciliated ends. Earthworms loosen soil and enrich it with humus, processing 250–600 t of soil per hectare a year; they are used to make vermicompost. In the sea live polychaetes (for example nereis), in fresh water leeches. Annelids are thought to have come from free-living flatworms.
Worked examples
A worm swallows soil and leaves castings: these soil pellets are rich fertiliser for plants. That is why farmers value soil with earthworms.
Medicinal leeches are bred at special farms and used only by a doctor for vascular diseases. Never put leeches from ponds on your skin.
Class activity
Make a drawing of the worm’s digestive tract with coloured pencils (red – blood vessels, yellow – gut). Safety: do not touch leeches in natural water; wash hands after handling worms.
Practice
1
What acts as the heart in the earthworm?
The 5–6 muscular ring vessels around the gullet.
2
If a worm is left on dry sand it soon dies. How is this linked to the way it breathes?
It breathes through its moist skin: oxygen dissolves in the moisture and passes into capillary blood. If the skin dries, oxygen cannot pass and the worm suffocates.
3
State the earthworms’ importance in nature.
They loosen and enrich soil with humus, let water and air in, give vermicompost and feed birds.
4
Why is the circulatory system called closed?
Blood does not leave into the body cavity but flows only inside vessels.