Otherwise they are clearly arthropods with modifications concerned with taking in liquid food and producing silk. Starfish have a central disc from which radiate a number of arms, usually five or a multiple of five. The mouth is in the centre of the underside, there is no head and any arm can act as the front end on the move. The gut is fairly rudimentary, the anus hardly used and there is no specialised excretory system, waste being absorbed by amoeboid cells. There is no brain, just a nerve ring around the mouth and a nerve cord in each leg connecting to a dorsal nerve net.
Sea urchins are just starfish with the arms sewn up.
As a matter of fact they show considerable resemblance to the chordates, which include you and me. The coelom in both chordates and echinoderms buds off from the gut. The echinoderm, incidentally has a brief bilaterally symmetrical stage before becoming secondarily asymmetrical. The simplest and most straightforward members of our own group, the chordates are small, inconspicuous animals of interest only to the Chinese who collect them by the ton for food. Characteristic of all chordates is the notochord, a flexible skeletal rod that develops from the roof of the gut.