A seastar arm cannot regenerate unless it has at least 1/5 of the central disk. (If you were to cut off an arm mid-way down, it wouldn't survive). Since most of the seastar organs are pentaradially symmetrical (they have the same parts repeated in each of the 5 sections), 1/5 of the seastar has pretty much everything it needs. There is only one small anus, but coarser residues are discharged back through the mouth anyway, so if it is eating, it can excrete waste. Liquid waste just diffuses out of the tube feet and papulae back into the water.