ABSTRACT
The internal and external modifications of the test determine the shape of the echinoid body. The points of attachment of the mesenteries of the digestive tract to the test cause internal modifications of the peristome, apical system, periproct and interambulacra. The stone canal is fastened separately to the madreporic plate. The radial canals with the symmetrical tube foot ampulla system and the muscular pharynx form the pentameric part of the test. The nonpentameric interambulacral part of the test develops into a pentameric counterpart to the ambulacral part of the globular or subconical sea urchin. The respiratory system is associated to pore pairs, and the nonrespiratory system is associated to single pores. The absence of the connective septa in the tube foot ampulla systems of the oral and ambital parts of the ambulacra, and the absence of the pharynx results in the loss of the pentamery of the test.
