Segmentation, the notochord and a key to the phyla

A body in segments

Look closely at an earthworm. Its body is a long row of rings. The division is not only on the outside. Inside, the body is divided by thin walls, and some organs repeat in each segment.

This is metameric segmentation, also called metamerism. The body is divided externally and internally into segments, with a serial repetition of at least some organs. It is seen in annelids, such as the earthworm. Arthropods and chordates are segmented too, though less obviously.

A ringed look on the outside alone does not make an animal segmented. The organs inside must repeat as well.

The notochord

Some animals form a special rod during their early life.

The notochord is a rod-like structure formed from the mesoderm on the dorsal side, the back, during embryonic development.

• Animals with a notochord are chordates.
• Animals without one are non-chordates. They include the Porifera, the coelenterates, the worms, the arthropods, the molluscs and the echinoderms.

In most adult vertebrates, the notochord is replaced by the backbone. In some chordates it stays all through life.

A key to the phyla

Put the features together, and they sort the animals in a few steps. The chart is called the broad classification of kingdom Animalia.

1. At the cellular level? Then it is Porifera.
2. If not, is it radial and diploblastic? Then it is Coelenterata or Ctenophora.
3. If it is bilateral and triploblastic, look at the cavity. None means Platyhelminthes. A pseudocoelom means Aschelminthes.
4. A true coelom leaves Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata. A notochord marks the chordates.

The echinoderms sit with the bilateral animals here, though the adults are radial. Their larvae are bilateral.

The features, phylum by phylum

Only three phyla here are segmented: Annelida, Arthropoda and Chordata. The first five phyla have no circulatory system. The annelids and every phylum after them have one. For the levels of organisation, look back at the first lesson. You will meet each phylum in the next two courses.

You should find that a sponge is named at the first question. A coelomate needs all four questions, and even then only the chordates are named alone.

Try it in Python

The function works like the chart. It returns the group as soon as an answer settles it.

Practise with new numbers

Common mistakes

• Calling any ringed animal segmented. Metamerism needs organs repeated inside, not only rings outside.
• Placing the notochord on the belly. It forms on the dorsal side, the back.
• Thinking the notochord comes from the ectoderm. It comes from the mesoderm.
• Putting echinoderms with the coelenterates. They are radial as adults, but triploblastic coelomates.

Remember these

• Metameric segmentation: segments outside and inside, with some organs repeated (annelids).
• Notochord: a rod from the mesoderm on the dorsal side of the embryo; chordates have it.
• Key: cellular level, then symmetry, then the cavity, then the notochord.
• Coelomates: Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata.

Check yourself

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

1. What is metameric segmentation?

2. From which germ layer does the notochord form?

3. On which side of the embryo does the notochord lie?

4. An animal is bilateral, triploblastic and has a pseudocoelom. Name its phylum.

5. Name two coelomate phyla that are not chordates.