Edrioasteroids

"Edrioasteroids lived from the Lower Cambrian to the Mississippian, when they suddenly disappeared.  Edrioasteroids have not been recorded outside of North America and Europe.  They are unique among the echinoderms.  The had little to no ability for locomotion, and they had no appendages.  The internal organs are enclosed in a central body cavity called a theca.  The "arms" or ambulacra, do not lie on top of the thecal plates, rather the ambulacral plates lie amidst and between the thecal plates.  There are 5 ambulacra, creating a star shape.  Some species may have lived attached to the hard sea floor, but most attached themselves to shells of other organisms such as  brachiopods."