Roundworms
Roundworms
Roundworms, or nematodes, probably evolved from the earlier flatworms about 548 million years ago. Like all other living things at this time, they lived in the ocean. They were the first kind of animal that was big enough so that some of its cells were not touching the ocean water.
To keep the cells that weren't touching the water supplied with oxygen and food, roundworms developed the beginnings of a circulatory system.
After several million years, some of the roundworms probably evolved into segmented worms about 545 million years ago. But there are still tens of thousands of different kinds of roundworms in the world, and roundworms are very common all over the world (but not as common as arthropods).
A roundworm inside a person's eye
Most roundworms still live in the ocean. But some roundworms have evolved to live on land, and many other roundworms live inside people, other animals, and even plants. According to the World Health Organization, about one in every four people on Earth has roundworms living inside them (and it's mainly people in North America and Europe).
Learn by doing - Roundworms
To find out more about roundworms, check out these books from Amazon.com or from your library:
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