Platyhelminthes and bilateral symmetry
Flatworms, with two matching halves.
Left and right
Look at your own body. A line down the middle gives a left half and a right half.
What do you think such a body plan makes easier for an animal?
What this lesson covers
The idea
Flatworms show bilateral symmetry (body divisible into two halves along one plane), allowing directional movement; their flat bodies allow gas diffusion, a single opening remains, and many are parasites attached by hooks and suckers.
Left and right
Look at your own body. A line down the middle gives a left half and a right half.
What do you think such a body plan makes easier for an animal?
- Moving in a direction
- Staying fixed in one place
- Taking in food through pores
Features of flatworms
Open each tab.
A step forward
Bilateral symmetry gives a head end and a tail end, so a flatworm can move in a direction. Its thin flat body lets gases diffuse without special organs. It still has one opening. Many flatworms live as parasites with hooks and suckers.
Flatworms show bilateral symmetry (body divisible into two halves along one plane), allowing directional movement; their flat bodies allow gas diffusion, a single opening remains, and many are parasites attached by hooks and suckers.
Notes
Flatworms have bilateral symmetry for directional movement, flat bodies for gas diffusion and a single opening; many are parasites with hooks and suckers.
Check yourself
What does bilateral symmetry mean?
What does bilateral symmetry allow?
Why do flatworms not need specialised respiratory organs?
How do parasitic flatworms attach to the host?
- The body has no head or tail. Bilateral symmetry comes with distinct head-tail and front-back regions.
- The body is fixed in one place. Bilateral symmetry allows better coordination of movement.
- The body can be divided into two halves along one plane — correct. Yes!
- Better coordination of movement, with directional activity — correct. Yes!
- Water to flow through pores. This describes sponges.
- A shell to form. The book links bilateral symmetry to directional movement.
- They have two openings. Flatworms still have a single opening for food intake and waste elimination.
- Their flat bodies allow efficient diffusion of gases — correct. Yes!
- They have a backbone. Flatworms have no backbone. The book says gases diffuse through the flattened body.
- With tentacles. Tentacles belong to cnidarians.
- With a hard shell. Hooks and suckers help them attach firmly to the host tissues.
- With hooks and suckers — correct. Yes!