Flatworms (Platyhelminthes) and planaria
Flatworms have a soft, flat, ribbon-like body with three layers of cells (triploblastic), bilateral symmetry and no body cavity (acoelomate). Because they are flat, oxygen reaches each cell by diffusion.
Planaria lives in clean fresh water under stones. It glides on tiny hairs. Its head has two eye spots that sense light and a small brain-like cluster of nerve cells. The mouth is on the underside and a tube (pharynx) can stretch out to eat. The gut is branched with one opening, so there is no anus. Flame cells manage water.
Planaria has great powers of regeneration: a cut piece can grow into a whole new worm. Many other flatworms, such as liver flukes and tapeworms, are parasites with hooks or suckers.
Roundworms (Nematoda) and ascaris
Roundworms are slender, unsegmented cylinders with tapered ends, a tough outer layer (cuticle), and a false body cavity (pseudocoelom). They have a complete gut: mouth, tube, anus. They only have long muscles, so they move by thrashing from side to side.
Ascaris is a parasite in the human small intestine. It is 15 to 35 cm long, and females are bigger than males (separate sexes). It eats digested food from the host and can make a child weak. Eggs leave in faeces. They get into another person through dirty hands, soil or food. Prevention: wash hands and food, drink clean water, use toilets, and take deworming tablets as advised.
Annelids (Annelida) and earthworm
Annelids have a body made of many repeated rings, segments, inside and out. They are triploblastic with a true coelom, bilateral symmetry and a complete gut.
Earthworm (about 100 to 150 segments) lives in moist soil. Its skin is wet and slimy, with many blood vessels, so it breathes through the skin. Each segment (except the first and last) has tiny bristles called setae that grip the soil. Near the front there is a swollen band, the clitellum, which makes a cocoon for eggs. A worm has both male and female organs (hermaphrodite) but two worms swap sperm.
It has a closed blood circulation with red blood and several pairs of heart-like vessels. A funnel-and-tube nephridium in most segments removes waste. The gut has a muscular gizzard that grinds soil. Earthworm castings make soil rich.
Compare the three
| Planaria | Ascaris | Earthworm | |
|---|---|---|---|
| Shape | Flat | Round | Round, in rings |
| Body cavity | None | False | True |
| Gut | Branched, one opening | Straight, mouth + anus | Straight, mouth + anus |
| Blood | None | None | Closed |
| Excretion | Flame cells | Canals | Nephridia |
| Life style | Free-living in water | Parasite | Free-living in soil |
Try it
In the 3D: go to the last step. For each worm, predict how many pieces will survive, then move the time slider to check.
At home (safe): after rain, look under a stone in the garden. Count the rings on a worm if you find one, then put it back gently in damp soil. Note that it keeps its skin wet.
Key formulas and definitions
- Flatworm (planaria): flat, no cavity, branched gut with one opening, flame cells
- Roundworm (ascaris): round, false cavity (pseudocoelom), straight gut, parasite
- Annelid (earthworm): segmented, true coelom, closed circulation, nephridia, clitellum
- All three: triploblastic and bilaterally symmetrical
Worked examples
1. Which of the three has no anus?
Planaria. Its branched gut has a single opening, the mouth, so waste leaves by the same route.
2. Why do the earthworm and ascaris both look like long tubes yet belong to different groups?
The earthworm is divided into rings and has a true coelom and blood vessels. Ascaris is not segmented and has only a false cavity.
3. How can planaria and ascaris cause different problems for people?
Planaria lives in ponds and is harmless. Ascaris lives in the human gut as a parasite and takes food from the host.
4. An earthworm is cut into two pieces behind the clitellum. Which piece is more likely to survive?
The front piece that has the head and clitellum can regrow the tail. The tail piece cannot make a new head, so it usually dies.
Common mistakes
- Calling all worms "annelids". Only segmented worms like the earthworm are annelids.
- Saying planaria has an anus. It has none.
- Thinking that cutting an earthworm makes two worms. Usually only the head part survives.
- Mixing up cavity types: planaria none, ascaris false, earthworm true.