Read and research the echinodermata, paying special attention to the 10 broad categories we have been focusing on with various animal groups: classification, body plan/symmetry, feeding/digestion, respiration/gas exchange, excretion, nervous system/response, reproduction, mobility/locomotion, circulation, ecological importance.
Check youtube for other useful videos or watch https://www.youtube.com/watch?v=ZS-ce5NnS-g again.