Symmetry, asymmetry, and dissymmetry in echinoids

The development of bilateral symmetry of echinoid test in the course of exocyclization and phenomena of dissymmetry in the structure of paired interambulacra and apical system are examined. Peculiar metamery is typical of ambulacra, some plates of which are characterized by the similarity symmetry. The operation of translation in each ambulacrum results in the identity of form; however, it does not provide equal coincidence of each plate. The considerable asymmetry of petaloids in some irregular echinoids results from different functions of tube feet in adjacent petaloid rows.