Three groups of worms
Worms have soft bodies with no legs and no backbone. Scientists put them in three groups.
- Flatworms (planarians, flukes, tapeworms): the body is flat. There is one opening to the gut, or none (tapeworm).
- Roundworms (Ascaris, hookworm): the body is a long round tube. The gut has two openings, a mouth and an anus.
- Annelids (earthworm, leech): the body is divided into many rings (segments). The gut runs from mouth to anus.
Parasites and their life cycles
A parasite lives on or inside another animal (the host) and takes food from it. A parasite often harms its host.
Parasites have special features: hooks and suckers to hold on, a thick skin that the host's juices cannot digest, and a huge number of eggs because many eggs die before they find a host.
Life cycle of the pork tapeworm:
- The adult tapeworm lives in the human gut and lays eggs. Eggs leave in the waste.
- A pig eats food that has eggs on it.
- The eggs hatch and the young worms go into the pig's muscle (meat) as cysts.
- A person eats raw or half-cooked pork. The cysts grow into a new adult worm.
The cycle is broken by clean toilets, washing hands and vegetables, and cooking meat well. Roundworms such as Ascaris spread through dirty food and water. Hookworms enter through bare feet from damp soil, so wearing slippers helps.
The earthworm
The earthworm is an annelid. Its body has more than 100 rings. A light band near the front, the clitellum, helps in reproduction. Each ring has tiny bristles that grip the soil. Muscles in a wave shorten and lengthen the body so the worm crawls.
The earthworm breathes through its moist skin, so it needs damp soil and comes out in the rains. It has a closed blood system with red blood and several tiny "hearts".
Why it is the farmer's friend: it digs tunnels that bring air and water to roots, it mixes soil layers, and its droppings (castings) add plant food.
Comparing the three groups
| Group | Body shape | Example |
|---|---|---|
| Flatworm | Flat, no rings | Tapeworm, liver fluke |
| Roundworm | Round tube, no rings | Ascaris, hookworm |
| Annelid | Round, with rings | Earthworm, leech |
Try it
In the 3D: press All three, Flat, Round and Ringed. In Life cycle, follow the egg and say what happens at each box. Then press Soil and see where air goes.
At home or in a garden: after rain, look for earthworms (do not hurt them). Count a few rings on one worm. Notice how it moves in waves. Wash your hands afterwards.
Key formulas and definitions
- Flatworm: flat body. Roundworm: round tube. Annelid: body with rings (segments).
- Parasite = lives on a host and takes food from it
- Tapeworm cycle: human gut - eggs in waste - pig eats eggs - cysts in meat - human eats meat
- Break the cycle: clean water, clean hands, cooked meat, footwear
Worked examples
1. A worm has a long round body, no rings and a gut with mouth and anus. Which group is it?
It is a roundworm (nematode).
2. Why does a tapeworm lay thousands of eggs?
Most eggs never reach a pig, so most die. Making a huge number gives a few of them a chance to complete the life cycle.
3. An earthworm has 120 rings. A garden has 15 earthworms. How many rings are there in all?
120 x 15 = 1800 rings.
Common mistakes
- Thinking all worms are the same kind. There are flat, round and ringed groups.
- Calling every worm a parasite. Earthworms are free-living and helpful.
- Saying earthworms breathe with lungs. They breathe through damp skin.
- Believing parasites are only on the outside. Many live inside the gut or muscles.