I am studying Amniota and I was wondering if someone could explain to me the following concepts. I know that the features of Chordata are the notochord, dorsal nerve cords, gill clefts (except for Tunicata); segmented muscles and a post-anal tail. I know that developmentally we are related to echinoderms. Can you explain the relationship between the Cordata and Echinodermata? Using the bird, reptile and mammal (or those that came from an aminotic egg) as an example can you explain the following:

a) Why these 3 (bird, reptile and mammal) are grouped together?

b) What advantages (from an evolution perspective) do these three (as representatives of chordata) have over echinoderms?

c) How the above advantages suit a certain environment or habitat?

Also can you some websites that might be helpful in understanding this concept.