What are the main characteristics of echinoderms?

I think you're reading out of a low-level-zoology book, there's actually 14 characteristics. But some of them are really hard to understand, i cant even understand some of them myshelf, but for some that i can understand, I'll type it in

1. Body unsegmented, radial

2.No head, no brain

3.It has a water-vascular system

4.Excretory organs abset( it cant pee)

5.And they can regenerate lost parts.

The other 9 are really hard to understand, i think you wouldn't really understand them