What is the major difference between echinoderms and other invertebrates?

The major difference between echinoderms and other invertebrates is that there are few examples echinoderms that do not live in ocean water and none live on land. There are examples of other invertebrates that live in both fresh and salt water and on land. Echinoderms also display radial symmetry as do the jellyfish and sea anemones. If you want to tag the one thing that echinoderms have that no other living invertebrate has, you would probably have to point to their water vascular system (see link).

If you have the McGraw Hill science textbook for 6th grade then you are all set. Look on pages A40-A48.