Echinoids (Echinoidea) in which a bilateral symmetry is superimposed on the radial symmetry and where the apical system no longer contains the periproct. In some groups, e.g. the spatangoids, the aboral portions of the ambulacra (see ambulacrum) terminate above the ambitus and are situated in recessed, leaf-like ‘petals’, the mouth is situated far forward, and behind the mouth there is a flattened area (the ‘plastron’) that bears flat spines and is formed from the posterior interambulacra (see interambulacrum).