Germ layers and the body cavity

Layers in the embryo

An animal starts as a single cell. As the embryo grows, its cells form sheets called germ layers. Every tissue of the adult comes from one of them.

• The ectoderm is the outer layer.
• The endoderm is the inner layer, which lines the gut.
• The mesoderm is a middle layer, which some animals have and others lack.

Diploblastic and triploblastic

• Diploblastic animals have two germ layers, the ectoderm and the endoderm. Between them lies the mesoglea, a jelly-like layer that is not made of true cells. The coelenterates, such as Hydra and jellyfish, are diploblastic.
• Triploblastic animals have a third layer, the mesoderm, between the other two. All animals from the flatworms onwards are triploblastic, right up to the chordates.

The mesoderm is important. Muscles, blood and much of the skeleton come from it in higher animals.

The body cavity

In many triploblastic animals, a space lies between the body wall and the gut. What matters is how this space is lined.

• Coelom. The cavity is lined by mesoderm on both sides: under the body wall and over the gut. Animals with it are coelomates: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
• Pseudocoelom. The cavity is not lined by mesoderm all round. Instead the mesoderm is present as scattered pouches between the ectoderm and the endoderm. Animals with it are pseudocoelomates, such as the roundworms, Aschelminthes.
• No cavity. The space between the body wall and the gut is filled with mesoderm cells. Animals like this are acoelomates, such as the flatworms, Platyhelminthes.

The pseudo- in pseudocoelom means false. It is a real space, but it is not a true coelom, because mesoderm does not line it.

Why a cavity helps

A fluid-filled cavity gives the gut and other organs room to move and grow. It also cushions them. In the earthworm, the fluid in the coelom presses against the muscles as the body crawls.

You should find that with two germ layers there is no mesoderm, so the cavity question does not arise. With three, the orange mesoderm fills the space, makes a few pouches, or lines a true coelom.

Try it in Python

The function names the body plan from two numbers: the germ layers and the kind of cavity.

Practise with new numbers

Common mistakes

• Calling the mesoglea a third germ layer. It is a jelly without true cells, so coelenterates stay diploblastic.
• Thinking every space in the body is a coelom. Only a cavity lined all round by mesoderm counts.
• Giving flatworms a pseudocoelom. They are acoelomate; the roundworms are pseudocoelomate.
• Calling pseudocoelomates diploblastic. They have three germ layers.

Remember these

• Diploblastic: ectoderm and endoderm, with mesoglea between (coelenterates).
• Triploblastic: ectoderm, mesoderm and endoderm (flatworms to chordates).
• Coelom: a cavity lined by mesoderm (annelids, molluscs, arthropods, echinoderms, hemichordates, chordates).
• Pseudocoelom: mesoderm in scattered pouches (roundworms). Acoelomate: no cavity (flatworms).

Check yourself

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

1. What lies between the two germ layers of a coelenterate?

2. Which germ layer do triploblastic animals add?

3. What makes a body cavity a true coelom?

4. Name a pseudocoelomate group.

5. Why are flatworms called acoelomates?