An annelid, or segmented worm
When did annelids evolve?
Roundworms probably evolved into segmented worms, or annelids, about 545 million years ago.
What’s a roundworm?
More about the Proterozoic
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What were annelids like?
The earliest annelids all lived in the ocean, as most kinds of annelid still do today. Some annelids live inside of sponges.
Wait, a sponge can be alive?
Annelids’ long bodies were shaped like a hollow tube, with a long tunnel running down the middle of their bodies. At one end was the mouth, and at the other end the anus, where the poop came out. Some annelids have stomachs, in the middle of the long digestive tube.
Evolution of stomachs
Earthworms: are they always helpers?
The circulatory system
The big difference between roundworms and segmented worms is that segmented worms have a real circulatory system with blood.
Circulatory system
Eyes and eyespots
Many segmented worms also have less developed eyes than roundworms – these segmented worms only have eyespots, like flatworms. But other segmented worms do have more developed eyes.
Annelids evolve into…
By about 510 million years ago, some of these segmented worms were already evolving into fish. But many of them stayed segmented worms, and there are still lots of segmented worms all over the Earth today, on land and especially in the water.
Go on to mollusks
More about fish
Bibliography and further reading about worms and leeches:
“By about 510 million years ago, some of these segmented worms were already evolving into fish.” This is incorrect. Fish and segmented worms are not closely related. Fish are deuterostomes and annelids are protostomes. Annelids are more closely related to arthropods than vertebrates.
Yes, technically fish are descended from the common long-ago ancestor of both segmented worms and fish. But this is not a technical account, just a general overview of evolution.