What are segmented worms? Annelids, evolution, and biology

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a brown earthworm on the grass

An annelid, or segmented worm

Roundworms probably evolved into segmented worms, or annelids, about 545 million years ago. The earliest annelids all lived in the ocean, as most kinds of annelid still do today. Some annelids live inside of sponges. Their 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.

Earthworms: are they always helpers?

The big difference between roundworms and segmented worms is that segmented worms have a real circulatory system with blood. 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.

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

Bibliography and further reading about worms and leeches:

Go on to mollusks
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By |2018-04-13T14:37:43+00:00May 27th, 2017|Biology|0 Comments
Cite this page: Carr, K.E. What are segmented worms? Annelids, evolution, and biology. Quatr.us Study Guides, May 27, 2017. Web. December 15, 2018.

About the Author:

Dr. Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram, Pinterest, or Facebook, or buy her book, Vandals to Visigoths.

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