Annelida — the ringed worms

This is the animal phylum in which the body cavity called the coelom first developed. Segmented worms such as the earthworm, the leech and the ragworm (Nereis) belong to it.

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Figure 13.18 — several species of Annelida: ragworm, earthworm and leech.

They live in damp land, sea water and fresh water environments.
The multicellular body is built from three cell layers, which is why they are called triploblastic.
They have long slender worm-like bodies. The body is divided into equal segments externally and internally, which is why they are called segmented worms.
The body shows bilateral symmetry.
Inside the body, between the body wall and the gut, is a fluid-filled cavity. This is the coelom. It prevents body movements from affecting the alimentary canal.
Some reproduce asexually and some sexually.

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Figure 13.19 — the external structure of an annelid: clitellum, segments, stiff bristles and a fertilised egg.

Why the coelom matters

With a fluid cavity between body wall and gut, moving does not squash the alimentary canal. Which is to say a worm can burrow and digest at the same time.