Flatworms, roundworms and annelids

The worms

Three phyla of animals are called worms. Their bodies look alike at first: long and soft, with no legs. But a closer look shows three quite different plans.

All three are bilaterally symmetrical and triploblastic: the body is built from three layers of cells. They differ in their body cavity.

• Platyhelminthes: flat, with no body cavity. They are acoelomate.
• Aschelminthes: round in section, with a false cavity. They are pseudocoelomate.
• Annelida: made of ringed segments, with a true cavity. They are coelomate.

Phylum Platyhelminthes: the flatworms

The body is flattened from top to bottom (dorso-ventrally). So they are called flatworms.

• Most are endoparasites: they live inside other animals, including human beings.
• They are bilaterally symmetrical, triploblastic and acoelomate, with an organ level of organisation.
• Parasites have hooks and suckers to hold on to the host.
• Some take in food made ready by the host directly through the body surface.
• Special flame cells help in osmoregulation and excretion.
• The sexes are not separate. Fertilisation is internal, and development passes through many larval stages.
• Some, like Planaria, have a great power of regeneration. A piece cut from the body grows into a whole worm.

Examples: Taenia (the tapeworm), Fasciola (the liver fluke) and Planaria.

The body of Taenia is a long chain of segments called proglottids. Ripe ones, full of eggs, break off and leave the host with the faeces.

Phylum Aschelminthes: the roundworms

The body is circular in cross-section. So they are called roundworms.

• They may be free-living, in water or on land, or parasites of plants and animals.
• They have an organ-system level of organisation, and are bilaterally symmetrical and triploblastic.
• They are pseudocoelomate: the body cavity is not fully lined by mesoderm.
• The alimentary canal is complete, with a well-developed muscular pharynx.
• An excretory tube removes wastes through an excretory pore.
• The sexes are separate: they are dioecious. Females are often longer than males.
• Fertilisation is internal. Development may be direct, with young that look like the adult, or indirect.

Examples: Ascaris (the roundworm), Wuchereria (the filarial worm) and Ancylostoma (the hookworm).

Wuchereria causes filariasis. Ascaris and Ancylostoma live in the human gut. Their eggs leave the body with the faeces and can reach new people through dirty hands, food or soil.

Phylum Annelida: the segmented worms

The word comes from the Latin annulus, "a little ring". The body surface is clearly marked out into rings called segments or metameres.

• Annelids live in the sea, in fresh water or on land. Most are free-living; some are parasites.
• They have an organ-system level of organisation and bilateral symmetry.
• They are triploblastic, metamerically segmented and coelomate: they have a true body cavity.
• Longitudinal and circular muscles help them move.
• Water-living annelids such as Nereis have side flaps called parapodia, which help them swim.
• The circulatory system is closed: the blood stays inside vessels.
• Nephridia help in osmoregulation and excretion.
• The nervous system has paired ganglia, joined by side nerves to a double ventral nerve cord.
• Nereis is dioecious. Earthworms and leeches are monoecious: one animal has both sexes.
• Reproduction is sexual.

Examples: Nereis, Pheretima (the earthworm) and Hirudinaria (the blood-sucking leech).

The Indian earthworm, Pheretima, has a body of about 100 to 120 segments.

You should find that $2$ females laying $200\,000$ eggs a day pass out $4\,000\,000$ eggs in $10$ days. This is why the eggs of roundworms are so common in soil near open drains.

Try it in Python

The program counts the eggs passed out by the roundworms in one person in a week.

You should see $4\,200\,000$ eggs. With one female the number falls to a third, $1\,400\,000$ eggs.

Practise with new numbers

Common mistakes

• Thinking that all worms are one group. Flatworms, roundworms and annelids are three different phyla.
• Calling roundworms coelomates. Their cavity is a pseudocoelom. Annelids are the first true coelomates here.
• Saying that every annelid has separate sexes. Nereis is dioecious, but earthworms and leeches are monoecious.
• Mixing up flame cells and nephridia. Flame cells belong to flatworms; nephridia to annelids.

Remember these

• Platyhelminthes: flat, acoelomate, flame cells, hooks and suckers, hermaphrodite; Taenia, Fasciola, Planaria.
• Aschelminthes: round, pseudocoelomate, muscular pharynx, dioecious; Ascaris, Wuchereria, Ancylostoma.
• Annelida: metameres, coelom, nephridia, closed circulation, parapodia in Nereis; Pheretima, Hirudinaria.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How does a tapeworm feed, though it has no mouth or gut?

2. Which phylum of worms has its sexes separate, with females longer than males?

3. Name the organs of excretion in a flatworm and in an annelid.

4. What are parapodia, and which annelid has them?

5. A student cuts a Planaria into three pieces and keeps them in clean pond water. What will she see after two weeks?