Kingdom Animalia: animals without a backbone

The invertebrates

Most animals have no backbone. They are the invertebrates. We sort them by how their bodies are built: how the cells are arranged, how the body is divided, and if there is a space inside the body.

The eight groups

1. Porifera (sponges). The body has many pores. The animals are fixed to a rock and do not move. The body has cells that do different jobs, but no tissues. Examples: Sycon, Spongilla.
2. Coelenterata (Cnidaria). The body has one opening and a cavity, and two layers of cells. Examples: Hydra, sea anemone, jellyfish.
3. Platyhelminthes (flatworms). The body is flat, like a leaf or a ribbon. They have organs, and many are parasites. Examples: planaria, liver fluke, tapeworm.
4. Nematoda (roundworms). The body is round and thin. Many are parasites. Examples: Ascaris, the filaria worm.
5. Annelida. The body is divided into segments like rings. They have organ systems and a true body cavity. Examples: earthworm, leech.
6. Arthropoda. The body has jointed legs and a hard outer covering. This is the largest group of animals. Examples: cockroach, butterfly, prawn, spider, scorpion.
7. Mollusca. The body is soft, often inside a hard shell. There is a muscular foot. Examples: snail, mussel, octopus.
8. Echinodermata. The skin has spines. The body has five parts around the centre. They live in the sea. Examples: starfish, sea urchin.

Levels of organisation

Clues that we use

• Symmetry. A starfish is the same all round: it has radial symmetry. A flatworm has a left side and a right side that are mirror images: it has bilateral symmetry.
• Segments. Annelids and arthropods have a body that is divided into parts.
• Jointed legs make an arthropod.

You should find that the number of lines of symmetry equals the number of arms. Five arms, $72^\circ$ apart, give five lines of symmetry.

Try it in Python

This program is a key. It names the group of an animal from one main clue.

Practise with new numbers

Common mistakes

• Putting all worms in one group. Flatworms, roundworms and annelids are three different groups.
• Calling a starfish a fish. It has no backbone. It is an echinoderm.
• Thinking a sponge is a plant because it does not move. It cannot make food, so it is an animal.
• Calling a spider an insect. Both are arthropods, but a spider has eight legs, and an insect has six.

Remember these

• Invertebrates have no backbone. There are eight main groups.
• Arthropoda has jointed legs and is the largest group. Annelida has ring-like segments.
• Mollusca has a soft body, often with a shell. Echinoderms have spiny skin and live in the sea.
• Radial symmetry: the same all round. Bilateral symmetry: a left side and a right side.

Check yourself

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

1. Which group has the earthworm?

2. Name one feature of the arthropods.

3. Which group has the sponges?

4. A starfish has five arms. How many lines of symmetry does it have?