Evolution of roundworms
Roundworms 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.
Tens of thousands of kinds
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. Roundworms are very common all over the world (but not as common as arthropods).
Where do roundworms live?
Most roundworms still live in the ocean. But some roundworms have evolved to live on land, and many other roundworms now live inside people, other animals, and even plants.