I n our backyards the main segmented worms, or annelids , are earthworms. If you've ever looked closely at earthworms, you've probably decided that they're pretty simple affairs, maybe even less sophisticated than snails and slugs. However, that's not necessarily the case. Their anatomy indicates that, in evolutionary terms, annelids are more advanced than mollusks. Annelids are segmented , with each segment bearing the same fundamental structures as all the others, though minor differences can occur between some segments.
Segmentation gives annelids greater mobility than mollusks have.
They don't have anything as fancy as a skeleton, or an ability to keep their bodies warm when the temperature drops. Also, the problem with their bodies drying out remains critical, despite the cuticle. They don't have very big brains, either. In other words, segmented worms stand about midway between the simplest and most complex of Earth's animals.